[jdk8u-dev] RFR: 8253939: [TESTBUG] Increase coverage of the cgroups detection code

[Jonathan Dowland]

This is a backport of 8253939  to jdk8u-dev for cgroups v2 support.

It's not clean: several `Files.writeString` → `Files.write(…,.getBytes())` changes needed.

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/159

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

Changes: https://git.openjdk.org/jdk8u-dev/pull/169/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8253939
  Stats: 147 lines in 2 files changed: 134 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/169.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/169/head:pull/169

PR: https://git.openjdk.org/jdk8u-dev/pull/169