gh-124570: ctypes: Run some Structure tests on Union as well (GH-124976)

- Move some Structure tests to test_structunion; use a common base
  test class + two subclasses to run them on Union too
- Remove test_union for now as it's redundant

Note: `test_simple_structs` & `test_simple_unions` are in the common
file because they share `formats`.