pythongh-117474: Skip GIL test in free-threaded build (python#117475)

In the free-threaded build, the GIL will typically be disabled so
`py-bt` will not show threads waiting on the GIL.

Lib/test/test_gdb/test_backtrace.py

@@ -49,6 +49,7 @@ def test_bt_full(self):
 
     @unittest.skipIf(python_is_optimized(),
                      "Python was compiled with optimizations")
+    @support.requires_gil_enabled
     @support.requires_resource('cpu')
     def test_threads(self):
         'Verify that "py-bt" indicates threads that are waiting for the GIL'