clang and flake8 have had some notable differences in the results

they provide across different (recent) versions.
Recent versions of clang treated what was supposed to be a syntax

error as part of an incomplete lambda, generating two errors instead of

the one we expected.
Different versions of flake8 represented column indexes indexes

differently for syntax errors, causing the latest Python 2 and Python 3

versions of flake8 to produce different results.
The clang test code has been updated to use different syntax to avoid

the lambda assumption, and the flake8 test code and assertions have

been updated to check better test code but to assert using a regex to

cover all bases.
That assertion uses assertRegex(). This is only in Python 3, and the

precursor, assertRegexpMatches(), is deprecated there. To avoid

reinventing the wheel, we're now conditionally using the common

unittest2 library of backports for Python 2.7.

Unit tests pass locally.
Checked results manually on a CI Docker image to try to verify behavior.