Match xfail behavior for pytest and unitttest with TAP spec

[mblayman]

This PR is intended to make the all the different xfail behaviors work (including pytest xfail and unittest.expectedFailure) and be consistent with the TAP specification.

Here's the truth table that I expect.

Failure type	Test result	TAP output
unitest.expectedFailure	PASS	ok 1 test_name # TODO unexpected success
unitest.expectedFailure	FAIL	not ok 1 test_name # TODO expected failure
pytest.mark.xfail(strict=False reason='the reason')	PASS	ok 1 test_name # TODO unexpected success: the reason
pytest.mark.xfail(strict=False reason='the reason')	FAIL	not ok 1 test_name # TODO expected failure: the reason
pytest.mark.xfail(strict=True reason='the reason')	PASS	not ok 1 test_name # unexpected success: [XPASS(strict)] the reason
pytest.mark.xfail(strict=True reason='the reason')	FAIL	not ok 1 test_name # TODO expected failure: the reason

Based on that truth table, there are a number of things in the plugin that need to change.

1. unittest.expectedFailure is backwards currently compared to the table. Fixing this will bring the behavior in line with TAPTestResult from tappy.
2. Strict unexpected success reports as TODO, but it should be treated as a non-TODO failure.

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Fixes #55 (eventually)

[mblayman]

@lazka, if you don't mind, could you take a look at the truth table in this PR description to see if you agree with my expected TAP output for each of the xfail scenarios?

I'll probably work on making the plugin match up with that table. I'd love to get a second pair of eyes if you have the time.

[lazka]

Looks good, except the "XPASS(strict)" I would expect to be "ok" (?)

[mblayman]

My rationale for treating XPASS as failing is that, by using strict, the user is opting into behavior to make sure the suite fails when something unexpectedly passes.

In my thinking, there's nothing to do and it would be inconsistent with their stated expectation if the TAP report treated it as an ok.

[lazka]

In my thinking, there's nothing to do and it would be inconsistent with their stated expectation if the TAP report treated it as an ok.

Ah, I missed that there is no TODO. yes, makes sense, thanks!

[lazka]

Thanks!