[jdk11u-dev] RFR: 8292206: TestCgroupMetrics.java fails as getMemoryUsage() is lower than expected

[Goetz Lindenmaier]

I backport this for parity with 11.0.20-oracle.

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Commit messages:
 - Backport 6ccee839580fd9dc4cd4941b44dbbe3105202561

Changes: https://git.openjdk.org/jdk11u-dev/pull/1868/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292206
  Stats: 32 lines in 3 files changed: 15 ins; 9 del; 8 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1868.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/1868/head:pull/1868

PR: https://git.openjdk.org/jdk11u-dev/pull/1868