Update vstest monorepo to 18.6.0

Author: renovate[bot]

CSQLY.Tests/CSQLY.Tests.csproj

@@ -19,7 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
     <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.abstractions" Version="2.0.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.1">