pytest.ini: add -ra to addopts

[blueyed]

This displays a short test summary, which is very useful.

[emmatyping]

This seems fine. Maybe it will help accomplish a long time goal for us to have the tests skipped listed in a nicer format :)

[JukkaL]

It's not clear to me how the test summary is useful. Can you give an example from the mypy test suite where this provides useful information?

[JukkaL]

In particular, when I tried running the tests with -k Enum -ra I got this which doesn't provide any useful context such as test name:

================================================ short test summary info ================================================
SKIP [1] /Users/jukka/src/mypy/mypy/test/data.py:220: <Skipped instance>

[blueyed]

It is more useful with failed/errored tests, where it has nodeids.
E.g. it would be useful to see here: https://travis-ci.org/python/mypy/builds/540634510#L608

[msullivan]

I'm going to close this since I don't think we get why this helps

[blueyed]

Fair enough - it (or rather the core of it) might become default anyway (pytest-dev/pytest#6454).