tests: kernel: timer_behaviour shouldn't use fabs

[RobinKastberg]

Currently the timer_behavior test uses fabs.
Since the C library functions being used in the kernel is restricted in Rule A.4 in the Coding Standard
We should probably not use these functions in kernel tests either.

To reproduce

scripts/twister -p qemu_cortex_m0 -T tests/kernel/timer/timer_behavior/ -x CONFIG_MINIMAL_LIBC=y
[...]
/workdir/zephyr/tests/kernel/timer/timer_behavior/src/jitter_drift.c: In function 'do_test_using':
/workdir/zephyr/tests/kernel/timer/timer_behavior/src/jitter_drift.c:336:22: error: implicit declaration of function 'fabs'; did you mean 'labs'? [-Werror=implicit-function-declaration]
  336 |         zassert_true(fabs(time_diff_us) < CONFIG_TIMER_TEST_MAX_DRIFT,
[...]

I am unsure if this is the "correctest" solution, but since it is simple and makes the test work in strictly more cases I think it should be fine?

There are tests FORCING minimal libc on similar kernel tests, possibly to uncover similar issues. Should I add it to this test as well?