chore(deps): update dependency microsoft.net.test.sdk to 18.6.0 (#176)

| datasource | package                | from   | to     |
| ---------- | ---------------------- | ------ | ------ |
| nuget      | Microsoft.NET.Test.Sdk | 18.5.1 | 18.6.0 |

Author: renovate[bot]

UnitTests/UnitTests.csproj

@@ -12,7 +12,7 @@
 
     <ItemGroup>
         <PackageReference Include="JunitXml.TestLogger" Version="8.0.0" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.5.1" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
         <PackageReference Include="xunit.v3.mtp-off" Version="3.2.2" />
         <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
         <ProjectReference Include="..\CSSParser\CSSParser.csproj" />