chore: Update vstest monorepo to 18.6.0

Author: renovate[bot]

src/Directory.Packages.props

@@ -5,7 +5,7 @@
   <ItemGroup>
     <PackageVersion Include="BenchmarkDotNet" Version="0.15.8" />
     <PackageVersion Include="IsExternalInit" Version="1.0.3" />
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" Version="10.0.300" />
     <PackageVersion Include="MinVer" PrivateAssets="All" Version="7.0.0" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.4" />