tests/qtest: Replace magic value by NANOSECONDS_PER_SECOND definition

Use self-explicit NANOSECONDS_PER_SECOND definition instead
of a magic value.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201011194918.3219195-5-f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/rtc-test.c

@@ -292,7 +292,7 @@ static void alarm_time(void)
             break;
         }
 
-        clock_step(1000000000);
+        clock_step(NANOSECONDS_PER_SECOND);
     }
 
     g_assert(get_irq(RTC_ISA_IRQ));