Log that master is connected and for which test_result this run is

Also log if master told us that we have nothing to do, and if the mode to run is local.

This should make it a bit more clear what is going on just by looking at
nxdtest log. See previous patch for more details and context.

For the reference: here is how updated output looks like in the normal case:

    date:   Thu, 09 Dec 2021 04:20:37 CET
    xnode:  slapuser7@rapidspace-testnode-001
    uname:  Linux rapidspace-testnode-001 4.9.0-16-amd64 #1 SMP Debian 4.9.272-1 (2021-06-21) x86_64
    cpu:    Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz
    # running for test_result_module/20211209-170FD3998

    >>> pytest
    $ python -m pytest
    ============================= test session starts ==============================
    platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.9.0, pluggy-0.13.1
    rootdir: /srv/slapgrid/slappart7/t/dfp/soft/47cc86af27d234f0464630f2a0d22a6f/parts/zodbtools-dev
    collected 46 items

    zodbtools/test/test_analyze.py .                                         [  2%]
    zodbtools/test/test_commit.py ..                                         [  6%]
    zodbtools/test/test_dump.py ...                                          [ 13%]
    zodbtools/test/test_restore.py ..                                        [ 17%]
    zodbtools/test/test_tidrange.py .............................            [ 80%]
    zodbtools/test/test_zodb.py .........                                    [100%]

    ========================== 46 passed in 9.15 seconds ===========================
    ok      pytest  12.433s # 46t 0e 0f 0s
    # ran 1 test case:  1·ok

/reviewed-by @jerome
/reviewed-on nexedi/nxdtest!15

nxdtest/__init__.py

@@ -189,10 +189,14 @@ def main():
 
         if test_result is None:
             # a test run for given name and revision has already been completed
+            emit("# master says: nothing to run")
             return
 
+        emit("# running for %s" % test_result.test_result_path)
+
     # master_url not provided -> run tests locally
     else:
+        emit("# local mode")
         test_result = LocalTestResult(tenv, run=args.run)
 
     # make sure we get output from subprocesses without delay.