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

tests/test-check-pyflakes.t

@@ -8,7 +8,7 b' run pyflakes on all tracked files ending'
   $ cat > test.py <<EOF
   > print(undefinedname)
   > EOF
-  $ pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+  $ $PYTHON -m pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   test.py:1: undefined name 'undefinedname'
   
   $ cd "`dirname "$TESTDIR"`"