Fix random test failure.
Tests methods were not properly isolated. The sequence testUpdateDeletedIDs() followed by testUpdateAddedIDs() caused the following failure:
17:42:32                  T E S T   F A I L U R E ! ! !
17:42:32 
17:42:32 Failed Test:  org.opends.server.backends.pluggable.DefaultIndexTest#testUpdateAddedIDs
17:42:32 Failure Cause:  java.lang.AssertionError: 
17:42:32 Expecting:
17:42:32  <[]>
17:42:32 to contain:
17:42:32  <[1, 2, 3, 4]>
17:42:32 but could not find:
17:42:32  <[1, 2, 3, 4]>
17:42:32 
17:42:32     org.opends.server.backends.pluggable.Utils.assertIdsEquals(Utils.java:39)
17:42:32     org.opends.server.backends.pluggable.Utils.assertIdsEquals(Utils.java:51)
17:42:32     org.opends.server.backends.pluggable.DefaultIndexTest.testUpdateAddedIDs(DefaultIndexTest.java:76)



DefaultIndexTest.java:
In setUp(), use @BeforeMethod annotation (rather than @BeforeTest)