Note that any serious definition of done includes a measure on the completeness of testing, including unit with coverage, integration and system (acceptance) testing. As always, any activity starts with a plan, such as a test plan, which should be easily derivable from the user stories.

Testability is one aspect we put great value on in our courses. That makes us choose rest as a transport mechanism between server and client app and using a Data Access Object (DAO) or Repistory style persistence architecture, because it lets you easily test the business code in isolation.