selftests/resctrl: Modularize resctrl test suite main() function

Resctrl test suite main() function does the following things
1. Parses command line arguments passed by user
2. Some setup checks
3. Logic that calls into each unit test
4. Print result and clean up after running each unit test

Introduce wrapper functions for steps 3 and 4 to modularize the main()
function. Adding these wrapper functions makes it easier to add any logic
to each individual test.

Please note that this is a preparatory patch for the next one and no
functional changes are intended.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>