Other tests in this file skip the test if libc_name is None. So I think it would make more sense to skip the test rather than fail in test_find(). I.e.

if not found:
    self.skipTest("Could not find c and m libraries")

If you are confident that find_library() will always find libc on AIX, perhaps you can suggest an extra test (or add to an existing test), to first check for the AIX platform, and only then fail if find_library() returned None.