[11u] RFR: 8249277: TestVerifyIterativeGVN.java is failing with timeout in OpenJDK 11

[Lindenmaier, Goetz]

Hi

This test times out on mac. I would like to increase the timeout. 
Please review:
http://cr.openjdk.java.net/~goetz/wr20/8249277-TestVerifyIterativeGVN_timeout-jdk11/01/

Oracle did a similar change in the closed repo:
  https://bugs.openjdk.java.net/browse/JDK-8248521 
but the change is not visible, so I crafted my own.

New bug, open 11u only:
https://bugs.openjdk.java.net/browse/JDK-8248521

Best regards,
  Goetz.