Can you come up with a test case? Put the module in Modules/Setup, then have a test case that tries importing it. The test case should check whether the module is in builtin_module_names, and, if it is, try importing it.

(Ideally, it should only make the module builtin if debug is enabled, but I can't see how that can be achieved. Perhaps some trickery in config.c could do that, and #ifdefing out the body of the module if debug is disabled).