add PI mutex tests
    
    for robust mutex tests, the code was already setup to be run more than
    once with different attributes (pshared); I've added PI the same way.
    
    for the main mutex tests, it would take a major refactoring to test
    and report errors well for both the normal and PI cases, so I just
    duplicated the source file with PI hard-coded on. this could be
    improved later if desired.