Update to XUnit v3

Author: Numpsy

src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj

@@ -22,13 +22,13 @@
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-    <PackageReference Include="xunit" Version="2.9.0" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.1">
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