RFR: 8372008: TestGetTotalGcCpuTime test failures on Windows (Some GC CPU time must have been reported)

[Jonas Norlinder]

Current trivial test for `TestGetTotalGcCpuTime` fails as some OSs may have a slower update rate for the underlying CPU time thread counters than the code anticipated, causing the test to fail which assumes that some work have been performed and the count should be non-zero.

Solution: All GCs we are testing are tracing GCs. Therefore, add some marking job to inflate the GC CPU time such that we can guarantee to observe a non-zero value.

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Commit messages:
 - Add some work for GC

Changes: https://git.openjdk.org/jdk/pull/28362/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28362&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372008
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28362.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28362/head:pull/28362

PR: https://git.openjdk.org/jdk/pull/28362