Don't check for exact mem_used in DGM tests

The D>M tests assume that mem_used will be *exactly* the same after
the test as before. When using accurate memory tracking this is
unlikely to be the case. Seeing as these checks aren't the main thrust
of these two tests, remove them.

Change-Id: Ieaed79ffc0daeb99721a9b994ba6dccce8bab175
Reviewed-on: http://review.couchbase.org/42463
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>