[libc] Fix sched_getscheduler return value

This patch fixes the return time for sched_getscheduler which was set to
always zero. The syscall documentation, however, defines:

On success, sched_getscheduler() returns the policy for the thread (a
nonnegative integer).

I also changed the return type for sched_setscheduler, but this change
didn't impact and test case.

This patch also removes the duplicated code from param_and_scheduler_test.cpp
and adds SCHED_BATCH and SCHED_IDLE to the tests.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D156700