oeqa/runtime/cases: make date.DateTest.test_date more reliable

The test uses the broken out time and can only handle about 59s of delay,
use a UNIX timestamp to allow for up to a 300s delay.

[YOCTO #14463]

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>