selftests: rtc: rtctest: add alarm test on minute boundary

[ Upstream commit 7b3027728f4d4f6763f4d7e771acfc9424cdd0e6 ]

Unfortunately, some RTC don't have a second resolution for alarm so also
test for alarm on a minute boundary.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>