Refactor common Blazor acceptance test code

[m-akinc]

Pull Request

🤨 Rationale

Part of #1976

👩‍💻 Implementation

I've created a new project in blazor-workspace to contain code shared between the Nimble and Spright acceptance test projects. I ran into issues with the page components not being found when running tests from the VS Test Explorer. Through trial and error, I discovered that the Setup instance needed to be created in the same assembly as the page components for them to be discovered. Normally this is governed by the AppAssembly property of the Router component (in App.razor), but that is not used by the test runner.

🧪 Testing

Tests pass.

✅ Checklist

• [x] I have updated the project documentation to reflect my changes or determined no changes are needed.

[m-akinc]

@msmithNI would you mind buddying/first-reviewing this?