tests: drivers: counter: nrf_rtc: Add delay in the test

Some Nordic platforms start counter with the delay and
test_short_relative_alarm test expects very early alarm,
just after starting the counter. Adding the delay which
is applicable only for counter based on Nordic RTC peripheral.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
diff --git a/tests/drivers/counter/counter_basic_api/src/test_counter.c b/tests/drivers/counter/counter_basic_api/src/test_counter.c
index cec3401..9636d9e 100644
--- a/tests/drivers/counter/counter_basic_api/src/test_counter.c
+++ b/tests/drivers/counter/counter_basic_api/src/test_counter.c
@@ -777,6 +777,10 @@
 	err = counter_start(dev);
 	zassert_equal(0, err, "%s: Unexpected error", dev->name);
 
+	if (IS_ENABLED(CONFIG_COUNTER_NRF_RTC)) {
+		k_busy_wait(1000);
+	}
+
 	alarm_cfg.ticks = 1;
 
 	for (int i = 0; i < 100; ++i) {