Improve tests

[dmytro-pryvedeniuk]

This PR brings some improvements for the tests. There are many changes but they are scoped to the tests only.

General changes in all tests:

• Dispose IDisposable items properly (IHost, CancellationTokenSource, db connections and commands)
• I want to address similar issues in non-testing code with the same approach: "a disposable should be disposed" - in a separate PR.

Marten-specific changes:

• Use DisableConventionalDiscovery with explicit handler registration and DurabilityMode.Solo in most of the tests
• Remove pre-generated handlers. It's not obvious how to update them in the test project
• Distribution tests are updated to include more checks, parallelize some tasks, simplify code.
• Tests marked as flaky were unmarked as they are stable enough.