Add integration tests for email

[benbucksch]

Objective

• Find regressions when we break one of the email protocols
• Test whether the email protocol implementations still work, with basic email reading functionality
• Test with various real world providers, like Office365, outlook.com, Yahoo, GMX, T-Online etc.
• Test with various on-premise servers, like Dovecot, Cyrus, Courier, Stalwart, mox

Implementation

1. Start with app/test/logic/Mail/basic.test.ts
2. Open the JPC connection from the test to the backend process
   • connectToBackend(), stopBackend()
   • Only for tests that need it.
   • But avoid starting and closing it all the time.
3. Start the backend
   • Start the backend before connectToBackend(), and shut down the backend after stopBackend() (i.e. under the same conditions as formulated in the last point)
   • Avoid starting the Mustang frontend window.
   • See startupBackend() in backend/backend.ts.
   • Ideally, the backend would run in a separate process, but maybe that's not necessary and we can start the backend in the test process.
4. Find a way to store the third-party passwords, without checking them into git. GitHub Action Secrets? Any better solutions?
5. Starting on-premise servers is out of scope for this issue.