RFR: JDK-8214003: Limit default test jobs based on memory size

[Erik Joelsson]

On machines with a small amount of memory compared to number of cpus, we 
see test failures, and sometimes very long test run times (due to 
swapping). Because of this we need to limit the default TEST_JOBS based 
on memory size, just like we do for build JOBS in configure.

This patch adds a limit at "memory in GB" / 2, which seems to alleviate 
the problem for our particular hosts.

Webrev: http://cr.openjdk.java.net/~erikj/8214003/webrev.01/

Bug: https://bugs.openjdk.java.net/browse/JDK-8214003

/Erik