Add 3 valid and 3 invalid parser tests

1. C Style Arrays: Arrays must be <Type>[] varName. The c style declaration that is allowed in Java is not allowed in MiniJava.
2. Hello World: Valid MJ program that outputs Hello World when run.
3. Inner classes: Inner classes are not allowed in MiniJava.
4. Receiver Parameter: An explicit receiver parameter is allowed in Java, but not in MJ.
5. Special Method: System.out.println() is a special method, that is not allowed to be called in MJ, but should be parsed correctly.
6. Wild Syntax: A few lines of code that is valid MJ code (but cannot be compiled with javac doe to unreachable code).