[#1697] Don't break PEP8 tests if test files have lines >79 chars

According to naming test methods [1] in our testing coding standards, we should
make the test names clear "...even if it means your method name gets really
long, since we don’t write code that calls our test methods there’s no
advantage to having short test method names".  Given this, we shouldn't
validate E501 on our test suite.

[1] https://ckan.readthedocs.org/en/latest/contributing/testing.html#naming-test-methods
[#1697] Revert blacklisted files removed in cc7e917

They still fail even without validating E501.
[#1697] Unblacklist test files that are passing PEP8
[#1697] Fix minor PEP8 issues
[#1697] Add comment explaining why we're ignoring E501 on tests
Merge branch '1697-dont-validate-e501-on-tests'
[#1697] Simplify _is_test() a bit
[#1697] Remove ckanext/resourceproxy/tests/test_proxy.py from blacklist

It's passing now.
[#1697] Fix PEP8's E303 on ckanext/resourceproxy/tests/test_proxy.py
Merge branch '1697-dont-validate-e501-on-tests'