tests: make test-extension.t's baduisetup.py test fail even if bdiff exists
A mercurial.bdiff module has existed at some point and when I have
built at an old version, some .pyc file or something seems to stick
around and test-extension.t fails. Let's remove the reference to bdiff
and simplify the test.
Differential Revision: https://phab.mercurial-scm.org/D1162
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors