RFR: 8212630: jtreg/gc/logging/TestMetaSpaceLog.java failed with Agent timed out

[Kim Barrett]

Please review this change to a test to attempt to eliminate very
intermittent timeouts in Oracle's regression testing.  (Note that more
recent failures don't involve agent timeouts, as the test has been
modified to no longer use agent mode.)

The test currently consists of one process which launches child
processes to test each of a set of GCs.  We suspect that approach may
result in the single main process exceeding the normal timeout period
when under high load or on a slow machine (or both).  If so, the
situation will only get worse if more GCs are added to the current
(incomplete) set.

We attempt to address this by instead having a separate main process
for each tested GC, with the GC to test specified by a command line
argument.

[It's possible there is some other underlying problem that has nothing  
to do with the specific test, in which case this change might actually
result in more frequent timeouts of this test.  If that happens, we'll
deal with it as a new problem, with more information than we have now,
possibly including extra timing information from JDK-8219149.]

CR:
https://bugs.openjdk.java.net/browse/JDK-8212630

Webrev:
http://cr.openjdk.java.net/~kbarrett/8212630/open.00/

Testing:
A few dozen local executions of the test.  That's not enough and
probably not in a configuration to encounter the problem -- failures
have been occurring on the order of 0.5% of the time under high load.