Update dependency Microsoft.NET.Test.Sdk to 18.8.0

Author: renovate[bot]

Directory.Packages.props

@@ -6,7 +6,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+        <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.8.0" />
         <PackageVersion Include="xunit" Version="2.9.3" />
         <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />
         <PackageVersion Include="Xunit.SkippableFact" Version="1.5.61" />