RFR: 8177967: Move TestGCOld to subdirectory

[Erik Helin]

Hi all,

this small patch just moves TestGCOld to its own subdirectory under 
hotspot/test/gc/stress (to follow the same convention as TestGCBasher 
and TestSystemGC).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8177967

Patch:
http://cr.openjdk.java.net/~ehelin/8177967/00/

Testing:
- make run-test TEST=hotspot/gc/stress/gcold
- JPRT (since I updated TEST.groups)

Thanks,
Erik