Use fixtures to implement xUnit setup/teardown

[nicoddemus]

As discussed in #517.

Implement setup_*/teardown_* using fixtures injected during collection.

During collection, TestCase subclasses with a setup_ and/or teardown_ methods would have this fixture injected in the class:

@pytest.fixture(autouse=True)
def _xunit_setup_teardown_function(self):
    if hasattr(self, 'setup_method'):
        self.setup_method(self)
    yield
    if hasattr(self, 'teardown_method'):
        self.teardown_method(self)

If neither method is present, then this fixture is not injected at all.

The same would be done for setUpClass and setUpModule.

This would solve the problems mentioned in #517 quite nicely.

---

As discussed, this change might introduce subtle errors so this might be better introduced with an option to revert to the old implementation.

This should no longer be a concern, because we will only handle pytest's methods this way.

EDIT: updated regarding #3094 (comment)

[pytestbot]

GitMate.io thinks the contributor most likely able to help you is @RonnyPfannschmidt.

[nicoddemus]

Looking at the code I realized that using a fixture for setUp and tearDown is not possible because they are called by unittest.TestCase.run while running the test, and not by pytest. The idea should still be possible to support setup_ and teardown_ methods though, as those are called by pytest (and actually are the topic of #517 anyways).

[nicoddemus]

As commented in #4091, yield-tests semantics are a roadblock to get this in, so this will need to be postponed to 4.1.

[nicoddemus]

Will be released in 4.2. 👍