RFR 8062450: Timeout in LowMemoryTest.java

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8062450
Webrev: http://cr.openjdk.java.net/~jbachorik/8062450/webrev.00

The test times out when "-XX:+ExplicitGCInvokesConcurrent" is specified. 
The test relies on the MemoryMXBean.gc() invocation returning only after 
the GC run has been completed. This is not true when 
"-XX:+ExplicitGCInvokesConcurrent" is set and the test timing is thrown 
off-balance resulting in deadlock.

The proposed solution is to add an appropriate "@requires" JTREG stance 
to prevent running this test with incompatible settings.

Thanks,

-JB-