test, fix invalid user-provided startup() method

[mattip]

(EuroPython sprints)
Fixes #760

Typically users will override the App.startup() method with one of their own. For most backends, this method must create a MainWindow. This PR refactors the call to startup so that after the call, a new _verify_startup() private method will be called which, for all the GUI backends, will enforce the condition. For DocumentApp app the verification is empty.

Additionally, the "dummy" test backend was not properly chaining calls so main_loop calls create which calls startup. This was changed too to make the new feature testable.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Looks good; a couple of minor suggestions provided as diffs.

The only other problem I can see is that there's no test of the DocumentApp analog of this process. It's technically covered, but there's no validation that it's actually happening.

[mattip]

I added a test for the DocumentApp path, and CI is passing.

[freakboy3742]

I've made a couple of minor tweaks to the error message wording, but otherwise this looks great! Thanks for the contribution.