tests/posix_semaphore: unify and increase allowed test4 margin

Previously, there was a very tight allowed margin (100us), then some
special cases for platforms for which the test would otherwise fail,
increasing the margin.
This turned out to be a maintanance burden, as each slightly special
board needed a PR adding the special case.

This commit sets a quite large margin (1000us, 0.1% of total delay),
which should be large enough to not trip over platform-induced timer
inaccuracies, but still verify that the module is using timers
correctly.

(This is not a timer accuracy test.)

tests/posix_semaphore/main.c

@@ -236,16 +236,16 @@ void test3(void)
     sem_post(&s1);
 }
 
-#ifdef BOARD_NATIVE
-/* native can sometime take more time to respond as it is not real time */
-#define TEST4_TIMEOUT_EXCEEDED_MARGIN (300)
-#elif CPU_FAM_NRF51
-/* nrf51 based boards needs a slightly higher margin value. Using 105us makes
- test4 result more reliable. */
-#define TEST4_TIMEOUT_EXCEEDED_MARGIN (105)
-#else
-#define TEST4_TIMEOUT_EXCEEDED_MARGIN (100)
-#endif /* BOARD_NATIVE */
+/*
+ * Allowed margin for waiting too long.
+ *
+ * Waiting too short is forbidden by POSIX, but is checked elsewhere.
+ *
+ * This allows waiting a little (0.1%) longer than exactly 1000000us.
+ * The value should be large enough to not trip over timer inaccuracies, but
+ * small enough to catch any fundamental problems.
+ */
+#define TEST4_TIMEOUT_EXCEEDED_MARGIN (1000)
 
 void test4(void)
 {