For example NEOCluster needs to shutdown gracefully, else there are
processes left for e.g. storage nodes and they dump somthing as below on
the terminal after tests completion:

    === RUN   TestLoad
    2020/10/21 14:33:00 zodb: FIXME: open ../zodb/storage/fs1/testdata/1.fs: raw cache is not ready for invalidations -> NoCache forced
    === RUN   TestLoad/py
    I: runneo.py: /tmp/neo445013868/1: Started master(s): 127.0.0.1:24661
    WARNING: This is not the recommended way to import data to NEO: you should use the Importer backend instead.
    NEO also does not implement IStorageRestoreable interface, which means that undo information is not preserved when using this tool: conflict resolution could happen when undoing an old transaction.
    Migrating from ../zodb/storage/fs1/testdata/1.fs to 127.0.0.1:24661
    Migration done in 0.19877
    --- PASS: TestLoad (0.75s)
        --- PASS: TestLoad/py (0.74s)
    PASS
    ok      lab.nexedi.com/kirr/neo/go/neo  0.749s
    (neo) (z-dev) (g.env) kirr@deco:~/src/neo/src/lab.nexedi.com/kirr/neo/go/neo$ Traceback (most recent call last):
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/tests/functional/__init__.py", line 182, in start
        getattr(neo.scripts,  command).main()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/scripts/neostorage.py", line 66, in main
        app.run()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 147, in run
        self._run()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 178, in _run
        self.doOperation()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 266, in doOperation
        poll()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/app.py", line 87, in _poll
        self.em.poll(1)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 155, in poll
        self._poll(blocking)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 253, in _poll
        timeout_object.onTimeout()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/lib/event.py", line 259, in onTimeout
        on_timeout()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/manager.py", line 207, in _deferredCommit
        self.commit()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/manager.py", line 193, in commit
        self._commit()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/sqlite.py", line 90, in _commit
        retry_if_locked(self.conn.commit)
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/neo/storage/database/sqlite.py", line 45, in retry_if_locked
        return f(*args)
    OperationalError: disk I/O error

This fixes

	=== RUN   TestLoad/py
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned tid unexpected: 0285cbacc06d3a4c  ; want: 0285cbacc06d3a4c
	    xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned buf != nil
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned tid unexpected: 0285cbad858bf2e6  ; want: 0285cbad858bf2e6
	    xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned buf != nil
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned tid unexpected: 0285cbad858bf2e6  ; want: 0285cbad858bf2e6
	    xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned buf != nil
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned err unexpected:
	        have: <nil>
	        want: neo://1@127.0.0.1:27145: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned tid unexpected: 0285cbacc06d3a4c  ; want: 0285cbacc06d3a4c
	    xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned buf != nil

NEO/py sends all zeros for empty data. Without this fix tests fail:

    --- FAIL: TestLoad/py (0.74s)
        xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
        xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c