Enable lazy fixtures with parametrized subfixtures and lazy fixtures in subfixtures

[YannickJadoul]

This PR should allow for more complex combinations of lazy fixtures:

• On the one hand, if a test/fixture is parametrized with one or more lazy_fixtures that require other parametrized fixtures themselves, this would now result in an error (see test test_lazy_fixtures_with_subfixtures) This should also fix issue #23, as demonstrated in the added test test_issues23.
• On the other hand, fixtures requiring subfixures with parametrized with lazy_fixtures would not have these lazy fixtures resolved (resulting in type errors; see test test_lazy_fixtures_in_subfixture). This is similar to PR #28, except that it resolves nested lazy_fixtures in the parameters instead of as return values (as I am assuming is the aim of this project?).

EDIT: Oh, yes, the first commit of this PR also fixes a few existing tests, after direct application of marks inside parametrize values were deprecated/removed.

[TvoroG]

@YannickJadoul, thanks a lot! It looks really good. I will definitely merge this PR but I need a little bit of time to get familiar with the new code and understand how it solves the limitations.

[YannickJadoul]

@TvoroG Great! Thank you :-) Thanks as well for making this library in the first place, by the way.

If you have any questions, do let me know.

I can give a quick, intuitive summary of the changes if that helps:

• Instead of calling fm.getfixturedefs, I call fm.getfixtureclosure and apply the parametrization for all of these, in a loop. This fixes the case where lazily-used fixtures have parametrized subfixtures.
• The one thing necessary for this is to have the "parent node" from where this fixture is called. On later versions, we can access metafunc.definition.parent. Before pytest 3.10.0, it's really hard to get hold of this (which is why you wrote get_nodeid before, I'm guessing?), but we can intercept that node from the pytest_pycollect_makeitem hook, since during this hook, pytest_generate_tests will be called.
• Finally, this other issue, of subfixtures parametrized with lazy fixtures, is resolved by an implementation of the pytest_fixture_setup hook, where we resolve request.param if it is a lazy fixture placeholder.

I am now also thinking that I will make the last commit into a different PR, since it has a different purpose. This should make it easier to review, as well :-)