Update Htmx.Tests project language version to preview

The Htmx.Tests.csproj file has been modified to use the latest language version ("preview"). This will allow the use of the most recent C# language features in this project.

Author: khalidabuhakmeh

test/Htmx.Tests/Htmx.Tests.csproj

@@ -3,6 +3,7 @@
     <PropertyGroup>
         <TargetFramework>net7.0</TargetFramework>
         <IsPackable>false</IsPackable>
+        <LangVersion>preview</LangVersion>
     </PropertyGroup>
 
     <ItemGroup>