Fix for Null Reference at TestEngineCommandGuess

In ENGINE :Added a property ( boolean ) IsTesting. It is set to True at
TestEngineCommandGuests ( this might have been handy earlier :D )
Added to DisplayGameIsWonScreen() && !this.IsTesting, to evade reading
from console for a nickname ( which returned Null for .Trim )
SecretNumber: added an if this.guesses == null.  at which .Clear
resulted in NullReference