Display doctest results in same format as test results in Dodona feedback

[pdawyndt]

Reporting of doctest results is compatible with the format used to display the Dodona feedback. It would therefore feel natural to show those results Dodona-style. This can be done by subclassing the class doctest.DocTestRunner, and overriding the methods report_start(), report_success(), report_unexpected_exception(), and report_failure().