feat: add test project to verify incremental source generators

[TimothyMakkison]

I've made the methods a little more generic so It can be used with different generators.

Follow up to #4563

[thomhurst]

Summary

Adds a new test project to verify incremental source generator behavior for the MethodAssertionGenerator.

Critical Issues

1. VSTest Dependency Violation (BLOCKING)
The new project uses xunit with xunit.runner.visualstudio and Microsoft.NET.Test.Sdk, which violates TUnit's critical rule #3: "No VSTest - Use Microsoft.Testing.Platform only. NEVER use Microsoft.VisualStudio.TestPlatform."

Location: TUnit.SourceGenerator.IncrementalTests/TUnit.SourceGenerator.IncrementalTests.csproj:15-16

Fix: Replace xunit with TUnit for the test framework. Look at existing test projects like TUnit.Assertions.SourceGenerator.Tests which correctly use TUnit, not xunit.

2. Missing Snapshot Test Updates
The PR adds incremental generator tests but doesn't include any .verified.txt snapshot files. Per rule #2: "Changes to source generator output or public APIs require running snapshot tests. Commit .verified.txt files. NEVER commit .received.txt."

While these are incremental behavior tests (not output tests), if any test failures produce .received.txt files, they should be reviewed and accepted as .verified.txt before merging.

3. Target Framework Inconsistency
The new project targets net10.0 while existing TUnit test projects use properties from TestProject.props. This creates inconsistency.

Location: TUnit.SourceGenerator.IncrementalTests/TUnit.SourceGenerator.IncrementalTests.csproj:6

Fix: Import TestProject.props at the top like other test projects:

<ImportProject="../TestProject.props" />

Then remove the explicit TargetFramework setting.

Suggestions

1. Consistency with existing tests: The project structure differs significantly from existing test projects. Consider following the pattern in TUnit.Assertions.SourceGenerator.Tests which uses TUnit and the standard test project structure.

2. Use public API from MethodAssertionGenerator: The tests reference MethodAssertionGenerator.BuildAssertion (line 20). This is good - it's the named pipeline step. Just verify this is intentionally public.

3. Consider adding documentation: Since this introduces a new testing pattern (incremental generator testing), adding a brief comment in TestHelper.cs explaining why this is a separate project could help maintainers.

Previous Review Status

Unable to retrieve previous review comments due to API scope limitations.

Verdict

⚠️REQUEST CHANGES - Critical rule violations must be fixed (VSTest dependency, missing import of TestProject.props)

[TimothyMakkison]

3. Target Framework Inconsistency

SelfRegisteredExtensions.cs(15, 9): [CS0234] The type or namespace name 'Framework' does not exist in the namespace 'TUnit.Engine' (are you missing an assembly reference?)

1. VSTest Dependency Violation (BLOCKING)

Tried switching to TUnit, not sure if they run was going to see if CI liked it

Updated the .sln file by blindly accepting everything, hopefully it's fine

[TimothyMakkison]

Can't get it to run with TUnit, I'm absolutely missing something here, but at least it runs 🤷