Used Ogooreck as API testing framework

1. Migrated all API tests to Ogooreck.
2. Removed all startup classes and used a single Program file and WebApplicationFactory for tests.
3. Aligned tests, and removed obsolete helpers.
4. Fixed issue with doubled registration of event handlers because of open-generic EventListener registration (replaced with IEventBus decorator).

Read more about Ogooreck in my blog post: Ogooreck, a sneaky testing library in BDD style.

See details in PR: #136.