RFR: 8283223: gc/stringdedup/TestStringDeduplicationFullGC.java#Parallel failed with "RuntimeException: String verification failed"

[Leo Korinth]

I have tried to reproduce the problem by adding load to the system but I have failed. In this patch I added the `systemLoad()` method and the `minMax()` method so that we can log the load when the problem appears. If the failure reappears (I can see no resent failures) we can use those methods to make the test pass on too high load.

`getSystemLoadAverage()` is a bit blunt and gives an average over one minute, but ought to work if we eventually would like to silence test failures on over-commissioned machines.

Do you think this is okay, or should I change the test to directly skip the testcase instead of adding the load log on failure?

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Commit messages:
 - 8283223: gc/stringdedup/TestStringDeduplicationFullGC.java#Parallel failed with "RuntimeException: String verification failed"

Changes: https://git.openjdk.org/jdk/pull/18327/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18327&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283223
  Stats: 44 lines in 1 file changed: 32 ins; 4 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/18327.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18327/head:pull/18327

PR: https://git.openjdk.org/jdk/pull/18327