Refactor the test suite

[fiseni]

This is a huge PR. Before we start with all the planned changes for version 9, I refactored the whole test suite and rewrote the tests from the ground up.

• Now that we have specification constructors as public, the specs are created within each test. This makes the tests self-contained and much easier to reason about. Removed all shared specifications that were used previously.
• Rewrote all the tests they were not very consistent. Also, I added tons of new tests.
• Added and utilized the Testcontainers.MsSql package. We no longer need the docker files, we can just run the dotnet test, simple as that. By default, we're also checking whether LocalDB is installed. If so, we're not even using containers at all. This can be overridden manually.

This is not done. I'll keep improving and adding tests while working on the features. The aim is to have 100% line and branch coverage.

[ardalis]

Can we remove this or uncomment?

[ardalis]

Uncomment or remove?

[ardalis]

Delete or uncomment

[ardalis]

Uncomment or remove

[ardalis]

I assume the commented full files are TODO...

[fiseni]

@ardalis In the next PR, I'll refactor the solution, and update the TFMs (add net9.0 and drop the unsupported ones). Also, I'll do a general cleanup (update the editorconfig and clean everything). There are some issues with "implicit usings" for .NET Framework consumers. I'll have to figure that part out. So, all those using statements will be consolidated.

As for tests, yes I've commented some of them. I'll be working on them as I work on the new features. At the end of this process, we'll have a consolidated and robust test suite.

This PR is already massive. So, let's merge it if possible, and I'll sort out everything along the way.

[ardalis]

If I approve you're welcome to merge - no need to wait for me. Sometimes I'll avoid merging myself in case you wanted to change anything before merging yourself.