[jdk11u-dev] RFR: 8278456: Define jtreg jdk_desktop test group time-based sub-tasks for use by headful testing.

[Amos]

- Backport for [JDK-8278456](https://bugs.openjdk.org/browse/JDK-8278456)

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Commit messages:
 - Backport 81c56c700a47c7f1b7855052de17eb88fac84674

Changes: https://git.openjdk.org/jdk11u-dev/pull/2164/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2164&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278456
  Stats: 23 lines in 1 file changed: 19 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2164.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2164/head:pull/2164

PR: https://git.openjdk.org/jdk11u-dev/pull/2164