Easier debugging for functional tests

[Pierre-Sassoulas]

Description

This merge request permits to :

• See what file was not found when a NoFileError is launched.
• See what is inside a list of FunctionalTest by being printed as FunctionalTest:test_name instead of <functional test object hexhash>.
• Have a better error message when a functional test fails because of a fatal pylint error (Ie: You're modifying the way functional tests are launched and you broke it).

Previously :

E   Failed: Wrong results for file "abstract_method_py3 ":
E          
E    Unexpected in testdata:
E      1: fatal

With this merge request :

E  AssertionError: Pylint analysis failed because of 'No module named a.abstract_method_py3'
E   assert 'fatal' != 'fatal'
E    +  where 'fatal' = Message(msg_id='F0001', symbol='fatal', msg='No module named a.abstract_method_py3', ...

• Permit to have subdirectory in tests/functional by using os.walk instead of os.listdir. There were hundreds of files all in the same directory and it was beginning to be hard to navigate. Now you can put any tests in any directory inside tests/functional.
• Move functional tests in a directory corresponding to the first letter of their name so navigation between them is more manageable tests/functional/example_tests.py => tests/functional/e/example_test.py. For functional test starting with too move them in a tests/functional/too directory so developers are aware it's a possibility and there aren't any magic going on forcing you to use the one letter directory approach.

Type of Changes

	Type
✓	🔨 Refactoring

[coveralls]

Coverage remained the same at 90.024% when pulling f4f34bd on Pierre-Sassoulas:easier-debugging-for-functional-tests into ac25a27 on PyCQA:master.

[Pierre-Sassoulas]

@PCManticore #2036 is based on this one, this is not absolutely necessary for it to work, but I would appreciate to not have to rewrite the commit tree :)

[AWhetter]

This has bugged me for a while. Seems like a good change to me!

[AWhetter]

This has bugged me for a while. Seems like a worthwhile change to me!

[PCManticore]

This is awesome! 😄 Thank you for tackling it @Pierre-Sassoulas

Merged it manually to get rid of the conflicts.