tests: timer_api: avoid compounding errors in imprecise conversions

When the timer frequency is not a multiple of 1000 converting between
ticks and milliseconds introduces error.  Avoid propagating the error
by converting divided values rather than dividing converted values.

Also compensate for observed rate differences between the busywait
clock and the timeout clock.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

tests/kernel/timer/timer_api/src/main.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <stdlib.h>
 #include <ztest.h>
 #include <zephyr/types.h>
 
@@ -538,6 +539,7 @@ void test_timer_user_data(void)
 void test_timer_remaining(void)
 {
 	u32_t dur_ticks = k_ms_to_ticks_ceil32(DURATION);
+	u32_t target_rem_ticks = k_ms_to_ticks_ceil32(DURATION / 2) + 1;
 	u32_t rem_ms, rem_ticks, exp_ticks;
 	u64_t now;
 
@@ -561,7 +563,19 @@ void test_timer_remaining(void)
 	zassert_true(rem_ms <= (DURATION / 2) + k_ticks_to_ms_floor64(1),
 		     NULL);
 
-	zassert_true(rem_ticks <= ((dur_ticks / 2) + 1), NULL);
+	/* Half the value of DURATION in ticks may not be the value of
+	 * half DURATION in ticks, when DURATION/2 is not an integer
+	 * multiple of ticks, so target_rem_ticks is used rather than
+	 * dur_ticks/2.
+	 *
+	 * Also if the tick clock is faster or slower than the
+	 * busywait clock the remaining time in ticks will be larger
+	 * or smaller than expected, so relax the tolerance.  3 ticks
+	 * variance has been observed on hardware where the busywait
+	 * clock is 0.7% slow and the 32 KiHz tick clock is 60 ppm
+	 * fast relative to a stable external clock.
+	 */
+	zassert_true(abs((s32_t)(rem_ticks - target_rem_ticks)) <= 3, NULL);
 
 	/* Note +1 tick precision: even though we're calcluating in
 	 * ticks, we're waiting in k_busy_wait(), not for a timer