Add C API tests for PySys_GetObject() and PySys_SetObject()

[serhiy-storchaka]

PySys_GetObject() will be changed in #106672, so we need tests.

Linked PRs

• gh-107735: Add C API tests for PySys_GetObject() and PySys_SetObject() #107736
• [3.12] gh-107735: Add C API tests for PySys_GetObject() and PySys_SetObject() (GH-107736) #107740
• [3.11] gh-107735: Add C API tests for PySys_GetObject() and PySys_SetObject() (GH-107736) #107741
• gh-107735: Move just added C API tests to better place #107743