tests/periph_timer_short_relative_set: improve test

Reduce the number lines to output by only testing for intervals 0..15
to speed up the test.

In addition, run each test case 128 repetitions (it is still faster
than before) to give some confidence the short relative set actually
succeeded.

tests/periph_timer_short_relative_set/main.c

@@ -75,33 +75,36 @@ int main(void)
     puts("\nTest for peripheral TIMER short timer_set()\n");
 
     printf("This test tries timer_set() with decreasing intervals down to 0.\n"
-           "You should see lines like 'interval <n> ok', followed by a success"
+           "You should see lines like 'interval <n>: OK', followed by a success"
            " message.\n"
            "On failure, this test prints an error message.\n\n");
 
-    printf("testing periph_timer %u, freq %lu\n", TEST_TIMER_DEV, TEST_TIMER_FREQ);
+    printf("testing periph_timer %u, freq %lu\n",
+           TEST_TIMER_DEV, TEST_TIMER_FREQ);
     timer_init(TEST_TIMER_DEV, TEST_TIMER_FREQ, cb, thread_get_active());
 
-    uint32_t interval = 100;
+    uint32_t interval = 16;
+    const unsigned max_repetitions = 128;
     while (interval--) {
-        uint32_t before = timer_read(TEST_TIMER_DEV);
-        timer_set(TEST_TIMER_DEV, 0, interval);
-        while(!thread_flags_clear(1)) {
-            uint32_t diff = (timer_read(TEST_TIMER_DEV) - before)
-                            & TEST_TIMER_MAX;
-            if (diff > TEST_MAX_DIFF) {
-                printf("ERROR: too long delay, aborted after %" PRIu32
-                        " (TEST_MAX_DIFF=%lu)\n"
-                        "TEST FAILED\n"
-                        "Note: This is currently expected to fail on most boards.\n",
-                        diff, TEST_MAX_DIFF);
-                while(1) {}
+        for (unsigned rep = 0; rep < max_repetitions; rep++) {
+            uint32_t before = timer_read(TEST_TIMER_DEV);
+            timer_set(TEST_TIMER_DEV, 0, interval);
+            while (!thread_flags_clear(1)) {
+                uint32_t diff = (timer_read(TEST_TIMER_DEV) - before)
+                                & TEST_TIMER_MAX;
+                if (diff > TEST_MAX_DIFF) {
+                    printf("ERROR: too long delay, aborted after %" PRIu32
+                            " (TEST_MAX_DIFF=%lu) on repetition %u\n"
+                            "TEST FAILED\n",
+                            diff, TEST_MAX_DIFF, rep);
+                    return EXIT_FAILURE;
+                }
             }
         }
-        printf("interval %" PRIu32 " ok\n", interval);
+        printf("interval %" PRIu32 ": OK\n", interval);
     }
 
     puts("\nTEST SUCCEEDED");
 
-    return 0;
+    return EXIT_SUCCESS;
 }