Update tests for the newcommand package, and update code

[dpvc]

This PR updates the test for the newcommand extension. In the NewcommandUtil.ts, the case where j < 0 does not occur, as j is reduced only on line 213, and that is only when hasBraces is true. For that to be the case, there would have had to have been an initial open brace, in which case line 206 would have called GetArgument(), which would find a matching close brace, so there must be at least two characters (the two braces), and j would be at least 2 before it is decremented. So it never gets below 0.