Fix test_gdb.test_pycfunction() for Python built with clang -Og.
Tolerate inlined functions in the gdb traceback.

When _testcapimodule.c is built by clang -Og, _null_to_none() is
inlined in meth_varargs() and so gdb returns _null_to_none() as
the frame #1. If it's not inlined, meth_varargs() is the frame #1.