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

[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). This patch alleviates that.

Thoughts?

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Commit messages:
 - 8253939: [TESTBUG] Increase coverage of the cgroups detection code for Java

Changes: https://git.openjdk.java.net/jdk/pull/609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=609&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253939
  Stats: 40 lines in 1 file changed: 27 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