Add separate test for asserting content of last edit message
Add separate test for asserting content of last edit message

Also remove 'last edit by' assertion from the older test as it
is now tested in a new separate test

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend