sunau, and wave modules.

Issue #7408: Forward port limited test from Python 2.7, fixing failing buildbot tests on BSD-based platforms.

for module_search_path. It was already the case on _Py_char2wchar() failure.

PATH_MAX is not declared on IRIX nor Windows.

the fullpath buffer, not PATH_MAX. fullpath is declared using MAXPATHLEN or
MAX_PATH depending on the OS, and PATH_MAX is not declared on IRIX.

the env_home buffer, not PATH_MAX+1. env_home is declared using MAXPATHLEN+1,
and PATH_MAX is not declared on IRIX.