Updates for String and List test cases

Added assertions for previously inconclusive test cases for List and String.

Added a bunch of new test cases for new String nodes:  String.Contains, String.StartsWith, String.EndsWith, String.IndexOf, String.Insert, String.Replace, String.TrimWhitespace

Following the existing test cases, this new set also checks for various input conditions:  normal input, multiple input (using Lists), empty string input, and invalid input (check for node warnings).