DERBY-3624: Missing deadlock in storetests/st_derby715.java

Make both tests wait until the other thread has executed the SELECT
statement and exhausted the ResultSet before they go on to the INSERT
statements that should deadlock.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1618821 13f79535-47bb-0310-9956-ffa450edef68