Refactor tests

[rfc2822]

This is a meta-issue. CC @ArnyminerZ @sunkup because we should discuss how to make the tests more reliable and more generally how we want our tests.

• Document how tests should look like / have a "sample test".
  • Define the preferred way to interact between mockk and Hilt.
  • See also https://dagger.dev/hilt/testing-philosophy.html
• Provide one preferred way (ideally with DI) of providing test data in the database.
  • Shouldn't interfere with other tests.
• Provide one preferred way (ideally with DI) of providing test accounts.
  • Shouldn't interfere with other tests.
  • In future, there will be accounts (in DB) and system accounts (Create system accounts dynamically from DB #843). The new way should already be designed for that.
  • See also [CI] Tests sometimes fail, probably because sync framework is called for short-lived test accounts #1273
• Provide one preferred way (ideally with DI) of providing test address books #1305
• DnsRecordResolverTest: testBestSRVRecord_MultipleRecords_Priority_Same not 100% deterministic (but should be) #1300