'Verify that "py-bt" indicates if a thread is garbage-collecting'

@unittest.skipIf(python_is_optimized(), 'Python    was    compiled    with    optimizations')
@unittest.skipUnless(thread, 'Python    was    compiled    without    thread    support')
def test_gc(self):

    cmd = 'from    gc    import    collect\nprint    42\ndef    foo():\n                collect()\ndef    bar():\n                foo()\nbar()\n'
    gdb_output = self.get_stack_trace(cmd, cmds_after_breakpoint=['break    update_refs', 'continue', 'py-bt'])
    self.assertIn('Garbage-collecting', gdb_output)
    gdb_output = self.get_stack_trace(cmd, cmds_after_breakpoint=['break    update_refs', 'continue', 'py-bt-full'])
    self.assertIn('Garbage-collecting', gdb_output)
