RFR: 8253939: [TESTBUG] Increase coverage of the cgroups detection code [v4]

[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 updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

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

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Changes: https://git.openjdk.java.net/jdk/pull/609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=609&range=03
  Stats: 147 lines in 2 files changed: 134 ins; 0 del; 13 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