RFR: JDK-8192995 run-test gtest should use all jvm variants, not just "server"

[Magnus Ihse Bursie]

The current implementation of gtest in run-test will use just the server 
variant of gtest. This is different from the old test/Makefile 
implementation, which used all available variants. (Or rather, those of 
server, client and minimal that were present.) The run-test 
implementation should do the same.

I have solved this by augmenting the gtest test descriptor. Now the 
fully qualified gtest test descriptor looks like this: 
"gtest:testname/variant", where "variant" is the JVM variant being 
tested. If the user specifies no variant, all available variants are 
tested. So "gtest:testname" will result in "gtest:testname/server" and 
"gtest:testname/client", if both client and server are present.

Bug: https://bugs.openjdk.java.net/browse/JDK-8192995
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8192995-run-test-gtest-all-jvm-variants/webrev.01

/Magnus