RFR (XS) 8220688: [TESTBUG] runtime/NMT/MallocStressTest.java timed out

[coleen.phillimore at oracle.com]

Summary: reduce number of threads and iterate rather than sleep.

Ran test 20x without timeout and faster now.  Left /timeout in test 
because it doesn't hurt.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8220688.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8220688

Thanks,
Coleen