Split 'dotnet test' classes in two directories (VSTest/MTP) (#50639)

Author: Youssef1313

src/Cli/dotnet/Commands/Test/CliConstants.cs

@@ -8,6 +8,7 @@ namespace Microsoft.DotNet.Cli.Commands.Test;
 internal static class CliConstants
 {
     public const string ServerOptionKey = "--server";
+    public const string HelpOptionKey = "--help";
     public const string DotNetTestPipeOptionKey = "--dotnet-test-pipe";
 
     public const string ServerOptionValue = "dotnettestcli";