[C#] Add Roslyn shapshot tests

[RReverser]

Description of Changes

Adds snapshot-based tests to the C# Roslyn incremental generator that handles attributes for C# module support.

Normally such code is emitted behind the hood, and can only be tested either via runtime tests - which might be insufficient - or by asking the compiler to emit those "fake" files via <EmitCompilerGeneratedFiles>true</...>, which puts unformatted outputs into a somewhat obscure folder and can be hard to analyse.

Instead, a simple example is now passed through the generator during normal dotnet test and snapshots are saved directly in Git, which should make it easier to spot improvements and regressions more quickly. In fact, it already helped to catch a bug with handling nested namespaces which I fixed in this PR as well.

In the future I want to extend those tests with more checks, in particular to ensure/improve cacheability of generators in IDE, but will do so in a separate PR.

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

1

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!

• Ran dotnet test from the very same PR.