test_os: cleanup test_internal_execvpe() and os._execvpe() mockup

 * Replace os.defpath instead of os.get_exec_path() to test also
   os.get_exec_path()
 * Use contextlib.contextmanager, move the mockup outside the class, and
   the mockup returns directly the call list object
 * Use two different contexts for the two tests
 * Use more revelant values and names
1 file changed