Add C API tests for the slice C API

[serhiy-storchaka]

There are no tests for PySlice_Check(), PySlice_New(), PySlice_GetIndices(), PySlice_GetIndicesEx(), PySlice_Unpack() and PySlice_AdjustIndices().

PySlice_Unpack() and PySlice_AdjustIndices() are covered indirectly, by tests for slicing of lists, bytes, arrays and memoryviews. PySlice_GetIndices() is not used in CPython at all, and PySlice_GetIndicesEx() is only used in _curses.

Linked PRs

• gh-158102: Add tests for the slice C API #158120