Allow "WithConcurrentAnalysis" in more places

[martin-strecker-sonarsource]

In the tests, we create a copy of the original test file and analyze it in parallel with the original one. To avoid compiler errors related to name clashes, we encapsulate the new code in a separate namespace like so "namespace {" + original + "}".

This doesn't work for

• file-scoped namespaces
• file header comments
• assembly attributes

because it causes compiler errors or analyzer failures on its own. When the above are needed in test files, the dev must remember to set .WithConcurrentAnalysis(false).

We need to insert the concurrent namespace in a smarter way to avoid the issues described above.