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.
Remove unneeded code from domain migration.

This patch removes unneeded code from be_migrate_thread_to_domain().
This function no longer needs to check the number of online
CPUs since the CPU mask for a given domain is now returned directly
from read_mapping().
Cosmetic: Fix error string to match fn name.

Patch cleans up an error message to match the
migrate-to-domain API.
read_mapping() breaks on NR_CPUS not div by 32

This patch fixes a bug in read_mapping(), which is used for
reading CPU and cluster (domain) mappings. read_mapping()
did not account for the case when NR_CPUS is not evenly
divisible by 32 (quite common when NR_CPUS < 32).