Use mono-spaced font for line-break detection tests

The line detection tests rely on text wrapping at a particular point in
order to function. Enforcing a monospace font and using the em value for
paragraph width measurements ensures (relatively) consistent behaviour
across different systems, irrespective of the system fonts available.
Merge pull request #575 from peitschie/line-tests

Use mono-spaced font for line-break detection tests