Support tests for files

[pdawyndt]

TESTed has a fixed list of predefined output channels (stdout, return, exit_code) but may also allow to specify additional output channels to check additional side-effects of running a submission.

The first use case I encountered is file checking. There are actual two use cases I already encountered:

• check if given file is not modified (assignments"om de hoek (opgave)" and "examenfeedback" in series 08 of the Computergebruik repo)
• check if a file was produces with an expected content (assignment "examenfeedback" in series 08 of the Computergebruik repo)

The main difference is that in the first case the student should only get a message when the file has been tampered with (as the content of the file is given), whereas in the second case (part of) the file content should be displayed in the feedback table. In latter case, the expected file content might be included in the test specification (for short files) or as a file reference (for long files), as was also suggested for stdin (#479).

Some support for file comparison might be included in TESTed itself: check if file is not modified; show difference between expected and actual file content (for short files).