DERBY-6306: XplainStatisticsTest.testSimpleQuery: BEGIN_COMP_TIME unexpectedly NULL

Clear the statement cache between test cases so that statements from
earlier test cases don't affect the results.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1518533 13f79535-47bb-0310-9956-ffa450edef68