This essentially tests tools/lint/python/flake8.py. Though it also adds
a basic framework for testing all the other linters as well. Getting this
added now will allow others to collaborate on adding more tests without
needing to get to 100% coverage for all linters right off the bat.
All python tests under tools/lint/test will run as part of the 'ml' task
on Linux, and the build task on Windows (OSX coverage is currently missing
for python tests).
The flake8 linter currently has a bug where the 'exclude' argument is
ignored. This is why we need to also exclude 'tools/lint/test/files' in
topsrcdir/.flake8, even though it is already listed in the
'mach_commands.py'. Other linters shouldn't need to do this, the exclusion
in 'mach_commands.py' should be good enough. See bug 1277851 for more
details.
MozReview-Commit-ID: 9ho8C83eeuj
