tests: consistently use pyflakes as a Python module stable

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

--- a/tests/test-check-pyflakes.t	Sat Jun 06 19:19:27 2020 +0800
+++ b/tests/test-check-pyflakes.t	Sat Jun 06 19:12:49 2020 +0800
@@ -8,7 +8,7 @@
   $ 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"`"