Make gevent.socket.getaddrinfo return enums on Python 3

Fixes #1310

Refactoring greentest

Fixes #1167 and fixes #1168

This lets us use the natural location for .coveragerc

By not actually recording anything for the tests. Presumably because they're executed as scripts, not as gevent.tests.whatever.

Travis is reporting src/gevent/test__* as uncovered, for some reason, but I don't get that locally.

Need to use an editable install on appveyor to find our test files; should probably loosen that restriction for downstream [skip travis]

In preperation for its removal.

Fixes #1305.