RFR: 8217332: JTREG: Clean up, use generics instead of raw types

[Leo Korinth]

Hi,

This webrev fixes lack of generics usage in the jtreg/gc folder.

This fix is somewhat based/dependent on "8214799: Add package 
declaration to each JTREG test case in the gc folder". That fix is out 
for review.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8217332

Webrev:
http://cr.openjdk.java.net/~lkorinth/8217332/00

Testing:
passed locally: open/test/hotspot/jtreg/:hotspot_gc

Thanks,
Leo