Update tests

[bart-degreed]

Description

I would like to give the integration tests in this project a big overhaul. Things I would like to address:

• Use WebApplicationFactory instead of TestServer
• Update Startup/Program from IWebHostBuilder to IHostBuilder
• Use a centralized set of typed deterministic (based on test name) Fakers and reuse them
• Use FluentAssertions for more readable tests and better failure messages
• Create a shared project with common building blocks, like executing a request, replacing an injected service etc.
• Have unit tests that assert exact json-text output (guards against breaking changes)

I'm also thinking about running tests in parallel.

• Each test class has its own database and factory, so classes can run in parallel
• Within a test class, use a unique database name (guid appended) and server instance, then run its containing test methods sequentially on that