Move all big test cases should into the folder `big`

Group 1: Add two full-program tests

With love from group 1 <3

Don't call the `decode()` method on a string object.

semantic and exec tests

some invalid semantic tests

Arrayutils fixes

more tests and fixes

System is a class and thus in a different scope.
There is no collision so the stdlib System should be used.

see https://github.com/mj3-16/mjtest/pull/23

needs to return