[server][testArmZabbixAPI] Reuse test event data to improve performance.

Previously the test event is loaded and parsed every request.
The new event acquisition method calls 'event.get' request many
time. As a result, it has made the test time longer.
This patch loads the test event once and reuses it to improve
performance.

The following is a measure time to execute
 test_verifyEventsObtanedBySplitWay() in my environment.

Prev: approx. 15.1 sec.
New : approx.  4.8 sec.