tests: benchmarks: sys_kernel: Reduce the number of loops.

The total number of loops being executed were taking far too long
to finish executing the test case. This was causing the timer handler
to execute in the middle of the test case. Thereby causing a test case
failure.
Now the number of loops have been reduced and this will make sure the
test case completes.

This change has no effect on the time being calculated by the
benchmark.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>