When adapting the test a while ago, Samir forgot that this one test
tested with multiple args. That was unnecessary, anyway, so removed
them.

Prevent unwanted FileNotFoundError caused by wrong test code shadow
actual msodde output

Instead of calling main() in current interpreter, fork a new one using
call_and_capture

This way, we get the output even in error case to simplify debugging

This is kind of a hack, hope this can be removed once other branch
is merged (see code comment)