Remove over-specification of task parameters from two tests

Don't override the defaults unless required.
Modernize rt_launch; remove internal API cruft

Switch rt_launch to use a more modern way to launch real-time tasks.
This makes rt_launch more flexible, makes it more similar to rtspin,
and also allows to let us get rid of a chunk of old, inflexible code
in liblitmus that wasn't really serving any useful purpose anymore.