Allow nestings tests within test groups

Signed-off-by: Olaf Kirch <okir@suse.de>
ni_process: groundwork for support of separate stderr/stdout

Signed-off-by: Olaf Kirch <okir@suse.de>
Support test grouping by nesting tests inside tests

Signed-off-by: Olaf Kirch <okir@suse.de>