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

test/jdk/TEST.groups defines the jdk_desktop group as a list of more-or-less functional sub-groups.
Some of these test groups have few tests and/or run in a very short time.
Others, notably jdk_awt may take almost 2 hours to run the headful tests

Whilst *headless* test tasks can be run in parallel on a system, *headful* tests must run sequentially.

So with a total test execution time of up to 3 hrs this is a lot.

The proposal here is that jdk_desktop will run the same set of tests as before, but it be split into 3 subgroups which are roughly expected to take equal amounts of wall clock time. These sub-groups will reference the existing test groups.

The primary benefit then is greater throughput when multiple machines are available to aborb the tasks in parallel. This will help us in that way at Oracle and should not affect any existing uses of the test targets since none are removed.