Previously, the markup parsing test would load a given markup file and
try to parse it several ways. It would return as soon as one of the
attempts failed — meaning that bugs only seen with non-nul-terminated,
or differently chunked, parse runs could never be caught.

Rework the tests so that all markup files are tested all ways, and we
assert that all ways of parsing them give the same result.

Signed-off-by: Philip Withnall <withnall@endlessm.com>