selftests/resctrl: Change the default limited time to 120 seconds

When testing on a Intel(R) Xeon(R) Gold 6254 CPU @ 3.10GHz the resctrl
selftests fail due to timeout after exceeding the default time limit of
45 seconds. On this system the test takes about 68 seconds.
Since the failing test by default accesses a fixed size of memory, the
execution time should not vary significantly between different environment.
A new default of 120 seconds should be sufficient yet easy to customize
with the introduction of the "settings" file for reference.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>