Add C API tests for PyObject_CopyData and PyBuffer_FillContiguousStrides #160688

Part of #94808.

Neither function has a wrapper in Modules/_testcapi or a test in Lib/test/test_capi. PyObject_CopyData has had real bugs (gh-143737) and there's an open overflow fix (gh-153690) with nothing to test it against. I'll add wrappers and tests along the lines of gh-158102, covering C/F order, non-contiguous sources, size mismatches and error paths.

Linked PRs

• gh-160688: Add tests for PyObject_CopyData() and PyBuffer_FillContiguousStrides() #160701