test/alarm: add delay tolerance

Because accuracy of timing to the microsecond is not guaranteed
in rte_eal_alarm_set, this function will not be called before
the requested time, but may be called a period of time
afterwards which can not be calculated. In order to ensure
test alarm running success, this patch added the delay time
before check the flag.

Signed-off-by: Qiming Yang <qiming.yang@intel.com>
Acked-by: Jing Chen <jing.d.chen@intel.com>