RFR: 8144913: Add extension point to execute_internal_vm_tests

[Erik Helin]

Hi all,

this small patch adds an extension point to execute_internal_vm_tests.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8144913

Webrev:
http://cr.openjdk.java.net/~ehelin/8144913/webrev.00/

Testing:
- JPRT

Thanks,
Erik