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

[Phil Race]

Define part1/part2/part3 for client jtreg tests as done for some other areas
See the bug report for some numbers on the potential benefits of this for faster testing.

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Commit messages:
 - 8278456: Define jtreg jdk_desktop test group time-based sub-tasks for use by headful testing.

Changes: https://git.openjdk.java.net/jdk/pull/6784/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6784&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278456
  Stats: 23 lines in 1 file changed: 17 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6784.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6784/head:pull/6784

PR: https://git.openjdk.java.net/jdk/pull/6784