tests: check availability of pyflakes by trying to import pyflakes module
Since e397c6d74652, we use the pyflakes module instead of the pyflakes
executable. As was pointed out during the review, the hghave check can be
rewritten to try to import the pyflakes module instead of spawning a new
subprocess.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed