tests: timer_behavior: use fabs() instead of abs()

abs() takes integer as argument but time_diff_us is of double.
So use fabs() instead.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

tests/kernel/timer/timer_behavior/src/jitter_drift.c

@@ -333,7 +333,7 @@ static void do_test_using(void (*sample_collection_fn)(void), const char *mechan
 		     "Standard deviation (in microseconds) outside expected bound");
 
 	/* Validate the timer drift (accuracy over time) is within a configurable bound */
-	zassert_true(abs(time_diff_us) < CONFIG_TIMER_TEST_MAX_DRIFT,
+	zassert_true(fabs(time_diff_us) < CONFIG_TIMER_TEST_MAX_DRIFT,
 		     "Drift (in microseconds) outside expected bound");
 }