chore(deps): update dependency xunit to v2.9.0

| datasource | package | from  | to    |
| ---------- | ------- | ----- | ----- |
| nuget      | xunit   | 2.8.1 | 2.9.0 |

Author: renovate[bot]

UnitTests/UnitTests.csproj

@@ -10,7 +10,7 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
-        <PackageReference Include="xunit" Version="2.8.1" />
+        <PackageReference Include="xunit" Version="2.9.0" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.8.1" />
         <ProjectReference Include="..\CSSParser\CSSParser.csproj" />
         <ProjectReference Include="..\CSSParser.ExtendedLESSParser\CSSParser.ExtendedLESSParser.csproj" />