Convert ServerNameCallbackTests to use pytest-style tests (#565)

* Convert ServerNameCallbackTests to use pytest-style tests

As well as pytest-ifying up the tests, remove a few redundant tests
and tidy up docstrings as per feedback in #563.

Addresses #340.

* Remove a stray ':py:obj:' in test docstring

* Remove _LoopbackMixin from TestServerNameCallback

Per @hynek feedback.  This test class only depended on one method,
which can be broken out as a separate function anyway -- I'll
gradually disassemble the loopback as I pytest-ify other tests.

* Re-wrap a few comments
1 file changed