Adding tests to exercise corner cases involving disowning.

[rwgk]

The most interesting corner case is in test_class_sh_disowning.py test_mixed. See comments there. A "maybe later" idea:

• For each overloaded function considered, in the first pass keep track if the call will raise ValueError "disowned" for one or more arguments. If this is true for the winning overload, raise a special exception before actually converting any arguments, to make the behavior predictable/portable (no arguments will be disowned).

This PR also fixing minor namespace naming inconsistencies across test_class_sh_*.cpp files.

[rwgk]

CI run produces 34 failing, 30 successful, with the failure below (seems to be all of the 34).

Question: Is the order in which arguments are converted from Python to C++ not deterministic (across multiple platforms)?

EDIT a few hours later: The answer is https://en.cppreference.com/w/cpp/language/eval_order — Thanks @wjakob for pointing this out!

    def test_mixed():
        while True:
            obj1a = m.Atype1(90)
            obj2a = m.Atype2(25)
            assert m.mixed(obj1a, obj2a) == (90 * 10 + 1) * 200 + (25 * 10 + 2) * 20
            obj1b = m.Atype1(0)
            with pytest.raises(ValueError):
                m.mixed(obj1b, obj2a)
            with pytest.raises(ValueError):
>               obj1b.get()  # obj1b was disowned even though m.mixed(obj1b, obj2a) failed.
E               Failed: DID NOT RAISE <class 'ValueError'>

[rwgk]

The CI is green, also with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT (#2879). This PR just affects tests, merging now to stay organized. This change will get reviewed internally. I'll open a new PR if there are requests for changes.