bpo-2661: Make mapping tests better usable for custom mapping classes. (GH-11157)

bpo-2661: Make mapping tests better usable for custom mapping classes. (GH-11157)

In test_fromkeys() the derived test class now supports all arguments in its
constructor so that the class to be tested can use its own constructor in its
fromkeys() implementation.

In test_mutatingiteration() the test fails as soon as iterating over a
dictionary with one entry and adding new entries in the loop iterates more
than once (to avoid endless loops in faulty implementations).

https://bugs.python.org/issue2661