tFile registers itself to tDB so that defer(tdb.close) that each test does,
automatically close all files before trying to unmount wcfs. However if an
error happens inside tFile constructor - before the place where
`tdb._files.add(t)` was previously located, tDB.close won't close tFile.f,
which will leave opened file on the wcfs -> wcfs unmount will fail with EBUSY,
and wcfs will be killed with huge backtrace.

If we register tFile early, the problem - even if it happens inside tFile
constor - will be handled gracefully: opened file closed and wcfs unmounted
clearnly - for example with the following short and easy to understand traceback

    (1.venv) kirr@deb9:~/src/wendelin.core$ python -m pytest -x wcfs/
    ============================================== test session starts ==============================================
    platform linux2 -- Python 2.7.13, pytest-4.6.11, py-1.9.0, pluggy-0.13.1
    rootdir: /home/kirr/src/wendelin.core
    collected 28 items

    wcfs/wcfs_test.py ..F

    =================================================== FAILURES ====================================================
    ________________________________________________ test_wcfs_basic ________________________________________________

        @func
        def test_wcfs_basic():
            t = tDB(); zf = t.zfile
            defer(t.close)

            # >>> lookup non-BigFile -> must be rejected
            with raises(OSError) as exc:
                t.wc._stat("head/bigfile/%s" % h(t.nonzfile._p_oid))
            assert exc.value.errno == EINVAL

            # >>> file initially empty
    >       f = t.open(zf)

    wcfs/wcfs_test.py:1080:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    wcfs/wcfs_test.py:322: in open
        return tFile(t, zf, at=at)
    wcfs/wcfs_test.py:477: in __init__
        mm.lock(t.fmmap, mm.MLOCK_ONFAULT)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    >   PyErr_SetFromErrno(OSError)
    E   OSError: [Errno 1] Operation not permitted

    wcfs/internal/mm.pyx:112: OSError
    --------------------------------------------- Captured stdout call ----------------------------------------------

    M: commit -> @at0 (03db288d33c34955)
    >>> Change history by file:

    --------------------------------------------- Captured stderr call ----------------------------------------------
    wcfs: 2020/10/08 19:13:12 zodb: FIXME: open file:///tmp/testdb_fs.1HuVZF/1.fs: raw cache is not ready for invalidations -> NoCache forced
    E1008 19:13:12.030532    3251 wcfs.go:2450] kernel FUSE (API 7.26) does not support explicit data cache invalidation
    E1008 19:13:12.030780    3251 wcfs.go:2450] -> performance will be AWFUL.
    E1008 19:13:12.030796    3251 wcfs.go:2450] -> you need kernel which includes git.kernel.org/linus/ad2ba64dd489.
    E1008 19:13:12.030809    3251 wcfs.go:2450] -> (Linux 5.2+, or nxd-fuse-dkms package installed from navytux.spb.ru/pkg)
    W: wcfs: kernel FUSE (API 7.26) does not support explicit data cache invalidation
    W: wcfs: -> performance will be AWFUL.
    W: wcfs: -> you need kernel which includes git.kernel.org/linus/ad2ba64dd489.
    W: wcfs: -> (Linux 5.2+, or nxd-fuse-dkms package installed from navytux.spb.ru/pkg)
    ----------------------------------------------- Captured log call -----------------------------------------------
    WARNING  ZODB.FileStorage:FileStorage.py:398 Ignoring index for /tmp/testdb_fs.1HuVZF/1.fs
    ====================================== 1 failed, 2 passed in 2.10 seconds =======================================