mlcp unittest reusing AssignmentManager

[karshuntsoi]

The following unit tests failed with my recent checkin:

testImportTransformMedlineZipFast(com.marklogic.contentpump.TestImportAggregate): expected:<[2]> but was:<[0]>
testImportDelimitedText_(com.marklogic.contentpump.TestImportDelimitedText): expected:<[1]> but was:<[0]>

The problem is the AssignmentManager did not get re-initialize after changing database in the test.