RFR: 8217385: JTREG: Clean up, make sure to close resources

[Leo Korinth]

Hi,

This clean up closes resources in some test cases in the jtreg/gc folder.

This fix is somewhat based/dependent on the rest of my "JTREG:" webrevs.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8217385

Webrev:
http://cr.openjdk.java.net/~lkorinth/8217385/00

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

Thanks,
Leo