tools/rtla: Add unit tests for utils.c

tools/rtla: Add unit tests for utils.c

Add unit tests for utility functions in src/utils.c using the Check
testing framework. The tests verify parse_cpu_set(), strtoi(), and
parse_prio() functions.

Unit tests are built conditionally when libcheck is available.
Run tests with 'make unit-test'.

The test framework uses the Check library which provides process
isolation for each test, preventing failures in one test from
affecting others.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Link: https://lore.kernel.org/r/20260119105857.797498-3-costa.shul@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>