Add xUnit tests readme

test/csharp/README.md

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+# xUnit Tests
+
+This tests are completely Linux specific.
+
+Every test class *must* belong to
+`[Collection("AssemblyLoadContext")]`. This ensures that PowerShell's
+AssemblyLoadContext is initialized before any other code is executed.
+When this is not the case, late initialization fails with
+`System.InvalidOperationException : Binding model is already locked
+for the AppDomain and cannot be reset.`
+
+Having every class in the same collection is as close to an xUnit
+global init hook as can be done.