RFR: JDK-8271721: Split gc/g1/TestMixedGCLiveThreshold into separate tests

[Thomas Stuefe]

Hi, may I please have reviews for this trivial change:

TestMixedGCLiveThreshold executes three tests in one by spawning off three child processes sequentially. In order to make the tests easier to analyze and to be able to execute them concurrently it would be helpful to have three subtests instead.

I also added diagnostic output.

Note: this was motivated while looking at test errors on ppc similar to JDK-8211081.

..Thomas

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/4968/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4968&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271721
  Stats: 35 lines in 1 file changed: 23 ins; 5 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4968.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4968/head:pull/4968

PR: https://git.openjdk.java.net/jdk/pull/4968