added tests for object_xref deletion
added tests for object_xref deletion

mjg17/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl,muffato/ensembl,danstaines/ensembl,at7/ensembl,Ensembl/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,willmclaren/ensembl,willmclaren/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,muffato/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,at7/ensembl