ROTS: SysTimer: Fix test timing issues

Use a busy loop with non-blocking Semaphore::wait(0) calls instead of a
single Semaphore::wait(osWaitForever) to improve time measurement
accuracy. Looping in Semaphore::wait(0) prevents the board from entering
sleep or deepsleep modes while waiting for the semaphore. By skipping
the overhead wakeup time, we get more accurate timings.

Author: Filip Jagodzinski

TESTS/mbedmicro-rtos-mbed/systimer/main.cpp

@@ -245,7 +245,10 @@ void test_handler_called_once(void)
     int32_t sem_slots = st.sem_wait(0);
     TEST_ASSERT_EQUAL_INT32(0, sem_slots);
 
-    sem_slots = st.sem_wait(osWaitForever);
+    // Wait in a busy loop to prevent entering sleep or deepsleep modes.
+    while (sem_slots != 1) {
+        sem_slots = st.sem_wait(0);
+    }
     us_timestamp_t t2 = st.get_time();
     TEST_ASSERT_EQUAL_INT32(1, sem_slots);
     TEST_ASSERT_EQUAL_UINT32(1, st.get_tick());