RFR (L) 8044140: [TESTBUG] Create NMT (Native Memory Tracking) tests for NMT2

[George Triantafillou]

Please review these new and modified tests for the redesigned Native 
Memory Tracking (NMT) functionality (8028541: Native Memory Tracking 
enhancement <https://bugs.openjdk.java.net/browse/JDK-8028541>).  NMT 
was redesigned internally to address scalability issues in the first 
implementation.  Existing functionality has not changed, with the 
exception of the removal of the "-AutoshutdownNMT" flag.

Please note that the following new stress tests are excluded with the 
JTREG @ignore tag:

test/runtime/NMT/MallocSiteHashOverflow.java
test/runtime/NMT/MallocStressTest.java

These tests can be run by using the jtreg override option 
"-ignore:run".  The new key "@key stress" was also added to these tests.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8044140
     Webrev: 
http://cr.openjdk.java.net/~gtriantafill/bug_8044140/webrev.00/ 
<http://cr.openjdk.java.net/%7Egtriantafill/bug_8044140/webrev.00/>
     Testing: JTREG to exercise the added tests

Thank you,

George