[jdk11u-dev] RFR: 8268906: gc/g1/mixedgc/TestOldGenCollectionUsage.java assumes that GCs take 1ms minimum

[SendaoYan]

Hi all,

This pull request contains a backport of commit [a0f32cb1](https://github.com/openjdk/jdk/commit/a0f32cb1406e4957e84befd9b68444adb662bd13) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Thomas Schatzl on 30 Jun 2021 and was reviewed by Kim Barrett, Albert Mingkun Yang and Leo Korinth.

The related failure was caught by dragonwell's CI. So we want to fix the intermittent failure. Only change the testcase, the risk is low.

Thanks!

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Commit messages:
 - Backport a0f32cb1406e4957e84befd9b68444adb662bd13

Changes: https://git.openjdk.org/jdk11u-dev/pull/2747/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2747&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8268906
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2747.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2747/head:pull/2747

PR: https://git.openjdk.org/jdk11u-dev/pull/2747