Refactor test suite

[asvetlov]

Now dependency tree for test_multidict.py is too complicated ever for me.
We need to slip it into several files, rewrite tests in pytest style and reduce test class dependencies.

Like:

READONLY_DICTS = [MultiDict, CIMultiDict, MultiDictProxy, CIMultiDictProxy]  # plus pure python classes

@pytest.parametrize(READONLY_DICTS)
def test_something(dct):
    pass

[webknjaz]

I'd add:

• eliminate use of pure unittest (inheriting from TestCase blocks using some of pytest features)
• move fixtures, which are copy-pasted across test modules to conftest.py

[webknjaz]

I believe this is fully addressed with #915 and 7513e6a.