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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed