pm selftest: rtc paranoia

Cope with a quirk of some RTCs (notably ACPI ones) which aren't guaranteed
to implement oneshot behavior when they woke the system from sleeep:
forcibly disable the alarm, just in case.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>