fcl-db: tests: when testing bufdataset use MergeChangeLog before saving test data into file to delete undo log. Tests which tests f.e. CancelUpdates are failed and they should not.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23630 3ad0048d-3df7-0310-abae-a5850022a9f2