rtc: move RTC_LIB_KUNIT_TEST to proper location

Move RTC_LIB_KUNIT_TEST under RTC_LIB so it is clear in the menu this is
part of the RTC subsystem.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210810212008.631359-2-alexandre.belloni@bootlin.com