[issue36824] Refactor str tests to reflect that str and unicode are merged in Python 3

[Daniel Fortunov]

Daniel Fortunov <pythonbugs at danielfortunov.com> added the comment:

PS opened here: https://github.com/python/cpython/pull/13172
I've tried to break down the changes into individual steps, with justification in commit messages. Happy to collapse these down into fewer commits before merge if preferred.

I haven't done the "Promote tests from the current `UnicodeTest` class to `CommonTest`" portion yet, but I'm running out of sprint time so I wanted to submit what I have.

I believe these changes are suitable for merge to master, but keen to hear feedback :-)

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