RFR: 8333446: Add tests for hierarchical container support

[Severin Gehwolf]

Please review this PR which adds test support for systemd slices so that bugs like [JDK-8217338](https://bugs.openjdk.org/browse/JDK-8217338) can be verified. The added test, `SystemdMemoryAwarenessTest` currently passes on cgroups v1 and fails on cgroups v2 due to the way how [JDK-8217338](https://bugs.openjdk.org/browse/JDK-8217338) was implemented when JDK 13 was a thing. Therefore immediately problem-listed. It should get unlisted once [JDK-8322420](https://bugs.openjdk.org/browse/JDK-8322420) merges.

I'm adding those tests in order to not regress another time.

Testing:
- [x] Container tests on Linux x86_64 cgroups v2 and Linux x86_64 cgroups v1.
- [x] New systemd test on cg v1 (passes). Fails on cg v2 (due to  JDK-8322420)
- [x] GHA

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Commit messages:
 - Fix comments
 - 8333446: Add tests for hierarchical container support

Changes: https://git.openjdk.org/jdk/pull/19530/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19530&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333446
  Stats: 489 lines in 8 files changed: 482 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/19530.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19530/head:pull/19530

PR: https://git.openjdk.org/jdk/pull/19530