wvtest.py: set wvtest to wvtest module if __main__

wvtest.py: set wvtest to wvtest module if __main__

Without this, references to wvtest.* may fail, which can be demonstrated
(for example) by adding a "raise AttributeError('foo')' to
_add_posix1e_acl just before the "if posix1e.has_extended(path)" test.
Then "./wvtest.py lib/bup/t/tgit.py" will fail in the end with:

  NameError: global name 'wvtest' is not defined

As a result, wvtest.py never prints a "! ... FAILED" message for the
test, and so "wvtest report" never realizes there was a problem, and
then "make check" exits with zero status.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>