chrt: (tests) Add new cases for custom slice on SCHED_{OTHER,BATCH}

chrt: (tests) Add new cases for custom slice on SCHED_{OTHER,BATCH}

Adds two new test cases setting --sched-runtime for SCHED_{OTHER,BATCH}.

The new custom slice tests are skipped for kernel versions < 6.12,
which do not have the feature. The existing chrt tests for
SCHED_{OTHER,BATCH} are skipped for kernel versions >= 6.12.

This is for two reasons:
 - the default sched_runtime value depends on target platform
 - without custom slice support, the value of sched_runtime is
   always zero for SCHED_{OTHER,BATCH}

Expected output with kernel version < 6.12:
   schedutils: chrt                           ...
                : batch                       ... OK
                : batch-custom-slice          ... SKIPPED
                : other                       ... OK
                : other-custom-slice          ... SKIPPED
		: deadline                    ... OK

Expected output with kernel version >= 6.12:
   schedutils: chrt                           ...
                : batch                       ... SKIPPED
                : batch-custom-slice          ... OK
                : other                       ... SKIPPED
                : other-custom-slice          ... OK
		: deadline                    ... OK

Signed-off-by: Petre Tudor <petre-ionut.tudor@arm.com>