RFR (XS) 8174734: Safepoint sync time did not increase

[coleen . phillimore]

Summary: allow safepoint time to be zero in the test

See bug for more details.

open webrev at http://cr.openjdk.java.net/~coleenp/8174734.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8174734

Thanks,
Coleen