Support xfailed and xpassed outcomes in RunResult.

[wimglenn]

Added support for xfailed and xpassed outcomes to the pytester.RunResult.assert_outcomes signature.

closes #3837

[RonnyPfannschmidt]

the linting failed, please use pre-commit to have our specific version of black sort out the code formatting

[wimglenn]

Yeah, my bad, I disabled it because the pre-commit hardcodes Python 3.6 but I only have 2.7 and 3.7 installed. Will fix manually..

[wimglenn]

@RonnyPfannschmidt rebased and linted

[RonnyPfannschmidt]

@asottile im wondering, can pre-commit be taught to use something more controllable there?

[asottile]

Upstream switched to just language_version: python3 🤷

[coveralls]

Coverage increased (+0.05%) to 92.598% when pulling 539a22c on wimglenn:runresult_xfail into 28aff05 on pytest-dev:master.

[RonnyPfannschmidt]

@asottile good point, what should pytest do, just the same?

[nicoddemus]

This should target features given it adds new capabilities to pytester, right?

(I want to take a good look at the code later, please don't merge yet 😁)

[wimglenn]

@nicoddemus Yes, I guess you're right - I've just switched the base branch.
Trying not to head-explode about how meta it is (tests for the tester for pytest and they run using pytester itself..) 🤕

[nicoddemus]

Trying not to head-explode about how meta it is (tests for the tester for pytest and they run using pytester itself..)

Heh! 😁

[nicoddemus]

Thanks @wimglenn!