You would currently need to delete those UnitTestInfoChange through a Python shell before you could delete the UnitTestInfo objects, and then finally the test.
Activate your virtualenv then:
python manage.py shell_plus
uti = UnitTestInfo.object.get(id=1605) # double check your ID
uti.unittestinfochange_set.all().delete()
uti.delete()
exit
Now you can delete your test through the admin.
Unfortunately this is quite cumbersome at the moment because we have tried to be careful with preventing people from accidentally deleting data. That said, I think we should change it so that deleting a UnitTestInfo object cascades and deletes the associated UnitTestInfoChange objects as well. I'll file an issue for this.
Randy