DERBY-6708 OOME when running ConstraintCharacteristicsTest with smaller heap

Patch derby-6708. Reduces the number of rows of the fixture
testManySimilarDuplicates from 40K to 4K. Removes the use of
in-memory database for all fixtures that use it.  In my setup (Ubuntu
12.04 64 bits, JDK 8u20) the test now runs fine with -Xmx128M, in ca.
70 seconds with "derby.system.durability=test".

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1619456 13f79535-47bb-0310-9956-ffa450edef68