selftests: rtc: rtctest: fix alarm tests

[ Upstream commit fdac94489c4d247088b3885875b39b3e1eb621ef ]

Return values for select are not checked properly and timeouts may not be
detected.

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>