RFR: 8147597: Always execute test functions in InternalVMTests

[Erik Helin]

Hi all,

this patch moves an check if G1 is enabled from InternalVMTests::run and
instead performs the check in the tests themselves. This makes the code
in InternalVMTests::run cleaner and also enables future enhancements.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8147597

Webrev:
http://cr.openjdk.java.net/~ehelin/8147597/00/

Testing:
- Running `make test-hotspot-internal` locally
- JPRT

Thanks,
Erik