RFR: 8253939: [TESTBUG] Increase coverage of the cgroups detection code [v3]

[Severin Gehwolf]

> Test only change. With [JDK-8253435](https://bugs.openjdk.java.net/browse/JDK-8253435) a test has been added on the hotspot side, but nothing for the Java Metrics code. Same for [JDK-8252359](https://bugs.openjdk.java.net/browse/JDK-8252359).
> 
> When JDK-8217766 got fixed cgroup factories with the detection logic didn't exist so were harder to test. This patch adds tests for them too.
> 
> Thoughts?

Severin Gehwolf has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  8253939: [TESTBUG] Increase coverage of the cgroups detection code

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/609/files
  - new: https://git.openjdk.java.net/jdk/pull/609/files/a122958f..44362211

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=609&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=609&range=01-02

  Stats: 107 lines in 2 files changed: 107 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/609.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/609/head:pull/609

PR: https://git.openjdk.java.net/jdk/pull/609