sysconf(_SC_NPROCESSORS_CONF) (via get_nprocs_conf) reflects
cpu_possible_mask, which is fixed at boot time, so querying it
repeatedly is unnecessary.

Replace multiple calls to sysconf(_SC_NPROCESSORS_CONF) with a single
global nr_cpus variable initialized once at startup.

`#pragma once` in timerlat_u.h is needed for pre-C23 compilers to avoid
redefinition errors.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Link: https://lore.kernel.org/r/20260306194953.2511960-2-costa.shul@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>

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>