Avoid prefixing variable names with namespace

[pdawyndt]

In this exercise we test the main call. Afterwards, we want to test the value of global variables. This test suite (where oracles have been removed) doesn't work

units:
- unit: "IO"
  cases:
    - script:
      - stdout: "Your roll is 1"
      - expression: "roll"
        return: 1
      - expression: "SIDES"
        return: 6

Because the variable roll is not available in the global namespace. Oddly enough the variable SIDES is available in the global namespace. We can resolve this by accessing the global variables through the submission namespace.

units:
- unit: "IO"
  cases:
    - script:
      - stdout: "Your roll is 1"
      - expression: "submission.roll"
        return: 1
      - expression: "submission.SIDES"
        return: 6

However, this exposes the submission namespace for students. This can be hidden somewhat using a description attribute, but the namespace keeps showing up in error messages and in the debugger (where it breaks the debugging session as there is not submission namespace over there).