Use fuzzy not exact cost comparison for the final tie-breaker in add_path.

Instead of an exact cost comparison, use a fuzzy comparison with 1e-10
delta after all other path metrics have proved equal.  This is to avoid
having platform-specific roundoff behaviors determine the choice when
two paths are really the same to our cost estimators.  Adjust the
recently-added test case that made it obvious we had a problem here.