tests/test-gendoc-zh_CN.t

tests: consistently use pyflakes as a Python module

We check availability of pyflakes as a module, and also running it for real as
a module. Only fair to test filterpyflakes.py working correctly when using
pyflakes as a module too.

This is a graft of a similar fix that ended up on default.

Differential Revision: https://phab.mercurial-scm.org/D8629

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_CN
  checking for parse errors