Verify Unity compatibility in the shared C# codegen

[RReverser]

Description of Changes

So far, the codegen tests verified that [SpacetimeDB.Type] and [SpacetimeDB.Table] markers result in expected generated methods, and that the generated code is valid and can be compiled successfully.

They did this by combining snapshot testing with a C# module example that has those attributes applied to a variety of types.

This works well enough, but now that C# codegen is split into two parts - BSATN.Codegen and Codegen - and the former is also used for C# clients like Unity, we want to make sure they work independently.

We also want to validate that the shared part that handles only the [SpacetimeDB.Type] attributes generates code compatible with Unity limitations - that is, it must be limited to .NET Standard 2.1 APIs and C# 9 syntax.

In this PR, I'm doing just that - creating two separate project fixtures. client one has limited .NET and C# versions, and only runs the BSATN.Codegen generator for the [Type] markers, whereas the server one does everything the old test did and tests modules with types, tables and reducers.

In the process I also simplified the way test projects are built by using a programmatic MSBuild API instead of manually building and collecting necessary DLLs into a compilation object. This brings test fixtures much closer to real-world projects, simplifies test maintainability, while still allowing to do all the low-level checks like those for codegen caching.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

How complicated do you think these changes are? Grade on a scale from 1 to 5,
where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff,
but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• dotnet test
• Write a test you want a reviewer to do here, so they can check it off when they're satisfied.

[bfops]

FYI @RReverser this is giving cryptic failures on the C# SDK tests, idk if that's expected.
Edit: Nvm that's not related to this PR.

[RReverser]

Edit: Nvm that's not related to this PR.

@bfops So is this one good to go then?

[kazimuth]

This makes sense, good to split the tests apart. The microsoft plumbing seems so much more pleasant than the proc-macro plumbing in Rust. I have no particular comments on the implementation, unfortunately.