[jdk11u-dev] RFR: 8266254: Update to use jtreg 6

[George Adams]

Backport required in order to bump to jtreg 6

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Commit messages:
 - Backport 3482cb87fd94e6445ca243722851daa90c00a2c8

Changes: https://git.openjdk.org/jdk11u-dev/pull/1241/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1241&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8266254
  Stats: 19 lines in 9 files changed: 0 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1241.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1241/head:pull/1241

PR: https://git.openjdk.org/jdk11u-dev/pull/1241