selftests: move RTC tests to rtc subfolder

Move the RTC tests out of the timers folder as they are mostly unrelated.
Keep rtcpie in timers as it only test hrtimers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>