tests: sched: Use SCHED_MULTIQ for native posix platform

To improve the code coverage on native posix, adding CONFIG_SCHED_MULTIQ
for scheduler api tests.

Extracting a separate prj_native_posix.conf file for the scheduler test,
which validates the "multi queue" scheduler on native posix.

Signed-off-by: Ajay Kishore <ajay.kishore@intel.com>