Correct the line numbers in the stacktraces for additional code TESTed might add
In preparation for #354
For example, if we add a line in the source code, this will not be displayed to the students, but the stacktraces are then wrong, so we now correct for this
This PR is mostly internal clean-ups: it is not visible to students, besides better stacktraces in some cases.
This PR is mostly internal clean-ups: it is not visible to students, besides better stacktraces in some cases.