Add @example(...).xfail(...) to check inputs which are expected to fail

[Zac-HD]

A classic error when testing is to write a test function that can never fail, even on inputs that aren't allowed or manually provided. @rsokl mentioned a nice design pattern for @pytest.mark.parametrize() of including an xfailed param to check that the test can fail; and after some discussion we agreed that this would be nice to have in Hypothesis too.

So: following #3516, I'd like to add an @example(...).xfail(condition: bool = True, *, reason: str = "", raises: type[BaseException] | tuple[...] = BaseException) - matching the interface of pytest.mark.xfail() (omitting the subset that doesn't make sense here). Implementation-wise, this will return self if condition is False, otherwise return an instance of a new XfailExample subclass which is known and handled by the execution logic in core.py. If the exception raised is not an instance of raises and our failure_exceptions_to_catch() it propogates as usual; if nothing is raised then we raise an error (using pytest.xfail() if available). Naturally we'll also support .via().xfail() and .xfail().via() 😁