RFR: 8333446: Add tests for hierarchical container support [v7]

[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

Severin Gehwolf has updated the pull request incrementally with four additional commits since the last revision:

 - Fix comment of WB::host_cpus()
 - Handle non-root + CGv2
 - Add nested hierarchy to test framework
 - Revert "Add root check for SystemdMemoryAwarenessTest.java"
   
   This reverts commit 7e8d9ed46815096ae8c4502f3320ebf5208438d5.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19530/files
  - new: https://git.openjdk.org/jdk/pull/19530/files/7e8d9ed4..a98fd7d6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19530&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19530&range=05-06

  Stats: 232 lines in 4 files changed: 167 ins; 35 del; 30 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