[3.15] gh-141510 Document and test frozendict class matching behaviour (GH-150799...

[3.15] gh-141510 Document and test frozendict class matching behaviour (GH-150799) (#151701)

gh-141510 Document and test frozendict class matching behaviour (GH-150799)

Frozendict has `_Py_TPFLAGS_MATCH_SELF` set so works correctly
with the single-arg class matching. However it isn't documented
in the list of classes this works with and it isn't tested.

The test is some way below the other similar tests but anything
else would need a large renumbering.
(cherry picked from commit fd53ae113911e5a7d83c04b08623df824f9d5d70)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>