Add tests for PyWeakref_* C API

[serhiy-storchaka]

Currently all tests for PyWeakref_* C API are implemented in a single C function test_weakref_capi() in Modules/_testcapimodule.c. it is not easy to add new tests, also the failure report is not very informative. The following PR adds separate module test.test_capi.test_weakref and wrappers in _testcapi and _testlimitedcapi modules to write tests in Python.

Linked PRs

• gh-151130: Add more tests for PyWeakref_* C API #151131
• [3.15] gh-151130: Add more tests for PyWeakref_* C API (GH-151131) #151140
• [3.14] gh-151130: Add more tests for PyWeakref_* C API (GH-151131) #151141
• [3.13] gh-151130: Add more tests for PyWeakref_* C API (GH-151131) (GH-151141) #151148