RFR: 8139256: Add Makefile target to run internal VM tests

[Erik Helin]

Hi all,

this small patch add a new top-level Makefile target:
test-hotspot-internal. The target is just an easier way to run the unit
tests in hotspot (the ones you execute by using the flag
-XX:+ExecuteInternalVMTests).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8139256

Webrev:
- root:
  http://cr.openjdk.java.net/~ehelin/8139256/root/webrev.00/
- hotspot:
  http://cr.openjdk.java.net/~ehelin/8139256/hotspot/webrev.00/

Thanks,
Erik