We can now selectively runxfail certain tests. This is useful for
accepting PRs into trezor-core:
1. trezor-core is going to get a pytest.ini that sets xfail_strict.
That means that if an `xfail`ed test actually passes, that will
break the test suite. So it will be visible when we implement
a feature for which tests exist.
2. To allow PRs to pass the test suite without touching python-trezor
directly, we add a new pytest.ini option: run_xfail.
This adds a list of markers which will ignore `xfail`.
So:
2.1 First, the python-trezor PR marks the tests with the name
of the feature. This commit already does that: Lisk tests
are marked `@pytest.mark.lisk`, NEMs are `@pytest.mark.nem`,
etc.
The tests will be also marked with `xfail`, because the
feature is not in core yet.
2.2 Then, the trezor-core PR implements the feature, which makes
the `xfail`ed tests pass. That breaks the test suite.
2.3 To fix the test suite, the core PR also adds a `run_xfail`
to `pytest.ini`: `run_xfail = lisk`.
(it can take a list: `run_xfail = lisk nem stellar`)
That will make the test suite behave as if the tests are not
`xfail`ed. If the feature is implemented correctly, the tests
will pass.
2.4 When the PR is accepted to core, the next step should be
a PR to python-trezor that removes the `xfail`s. After that,
we should also remove the `run_xfail` option, just to be tidy.
