jtreg should support a more flexible notion of "test groups".

Proposed spec:

1. In TEST.ROOT only (not in */TEST.properties)
   groups = file ... [file] ...
[file] is quietly ignore if file does not exist

2. In group files (specified as above)
   groupname = [-]dir ... [-]file ... +group...
A group is a name of collection directories, files, optionally excluding selected dirs and files, and nested groups, that together form a set of tests to be executed. [note, set implies no duplicates]

3. Groups can be specified on command line as +group