bpo-40334: Rewrite test_c_parser to avoid memory leaks (GH-19694)

bpo-40334: Rewrite test_c_parser to avoid memory leaks (GH-19694)

Previously every test was building an extension module and
loading it into sys.modules. The tearDown function was thus
not able to clean up correctly, resulting in memory leaks.

With this commit, every test function now builds the extension
module and runs the actual test code in a new process
(using assert_python_ok), so that sys.modules stays intact
and no memory gets leaked.