Refactor and improve integration test fixtures.

[fiseni]

Closes #359. More details on the related issue.

This PR contains the following improvements.

• Created a brand new DatabaseFixture. We'll create a unique database per each instance, and we'll delete the database on disposal.
• We seed the database with test data (we actually write data, and not using HasData).
• The tests won't reuse a shared DbContext, instead, we create a separate context for each test. This will lead to more reliable tests (especially while testing Include features).
• We can define collection fixtures now. All tests part of the same collection will work against the same database. For now, we have a single ReadCollection that is utilized for all repository read actions.
• For write actions, either we define a separate collection or utilize the ClassFixture instead.
• The fixtures for EF Core and EF6 are consistent now.
• Other improvements and cleanup.

[fiseni]

Note to self: The connection to the database fails. It happens randomly for a given project, usually 2 of them succeed completely and 1 one of them fails (and they shift randomly). I suspect now that we're creating DbContext for each test separately, we face connection pool exhaustion perhaps?

Btw, locally there is no issue at all; neither running from Test Explorer, nor through cli, nor through Docker. It has to do something with the environment in CI.

System.Data.SqlClient.SqlException : Connection Timeout Expired.  The timeout period elapsed while attempting to consume the pre-login handshake acknowledgement.  This could be because the pre-login handshake failed or the server was unable to respond back in time.  The duration spent while attempting to connect to this server was - [Pre-Login] initialization=20164; handshake=24;