Set RootNamespace in the MSTest project.

code/TemplateStudioForWinUICs/Templates/Test/App.MSTest/Param_ProjectName.Tests.MSTest/Param_ProjectName.Tests.MSTest.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>net6.0-windows10.0.19041.0</TargetFramework>
+    <RootNamespace>Param_RootNamespace.Tests.MSTest</RootNamespace>
     <Platforms>x86;x64;arm64</Platforms>
     <IsPackable>false</IsPackable>
     <ImplicitUsings>enable</ImplicitUsings>