gh-91960: Skip test_gdb if gdb cannot retrive Python frames (#108999)

Skip test_gdb if gdb is unable to retrieve Python frame objects: if a
frame is "<optimized out>". When Python is built with "clang -Og",
gdb can fail to retrive the 'frame' parameter of
_PyEval_EvalFrameDefault(). In this case, tests like py_bt() are
likely to fail. Without getting access to Python frames,
python-gdb.py is mostly clueless on retrieving the Python traceback.
Moreover, test_gdb is no longer skipped on macOS if Python is built
with Clang.