[jdk8u-dev] RFR: 8284102: [TESTBUG] [11u] Retroactively add regression test for JDK-8272124

[Jonathan Dowland]

Backport of 8284102 for jdk8u-dev for cgroups v2 support. Not clean, needed `Files.writeString` adjustment. Test builds and passes for me afterwards.

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/178

Commit messages:
 - adjust Files.writeString→write for jdk8u
 - 8284102: [TESTBUG] [11u] Retroactively add regression test for JDK-8272124

Changes: https://git.openjdk.org/jdk8u-dev/pull/179/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=179&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284102
  Stats: 53 lines in 1 file changed: 53 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/179.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/179/head:pull/179

PR: https://git.openjdk.org/jdk8u-dev/pull/179