tests/periph_rtc: delay RTC init

Just like `tests/periph_rtt`, delay the RTC init so that if init gets stuck
it doesn't get stuck in early boot but inside the test.
This makes debugging easier.

tests/periph_rtc/Makefile

@@ -2,6 +2,8 @@ include ../Makefile.tests_common
 
 FEATURES_REQUIRED = periph_rtc
 
+DISABLE_MODULE += periph_init_rtc
+
 USEMODULE += xtimer
 
 include $(RIOTBASE)/Makefile.include

tests/periph_rtc/main.c

@@ -74,6 +74,8 @@ int main(void)
     printf("This test will display 'Alarm!' every %u seconds for %u times\n",
             PERIOD, REPEAT);
 
+    rtc_init();
+
     /* set RTC */
     print_time("  Setting clock to ", &time);
     rtc_set_time(&time);