🔧 Adds default implcit namespace for tests

tests/Directory.Build.props

@@ -10,6 +10,10 @@
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
 
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
   <ItemGroup>
     <AssemblyAttribute Include="System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage" />
   </ItemGroup>