Add test to commands remove repl

Add test command to the common commands output. Remove repl, since the latest bits don't have it anymore. 

Fix #1287

src/dotnet/commands/dotnet-help/HelpCommand.cs

@@ -26,8 +26,7 @@ Common Commands:
   build         Builds a .NET project
   publish       Publishes a .NET project for deployment (including the runtime)
   run           Compiles and immediately executes a .NET project
-  test          Executes tests in a test project
-  repl          Launch an interactive session (read, eval, print, loop)
+  test          Runs unit tests using the test runner specified in the project
   pack          Creates a NuGet package";
         public static readonly string ProductVersion = GetProductVersion();