added test case for erroneous hashtag linkification

failed to find a solution so far - experimenting with variations of the
pattern for the hashtag `replace` call in `linkify` led nowhere

couldn't get difftest[1] to work, so using `compare` as a poor man's
version

clearly I have no idea what I'm doing...

[1] https://github.com/brentonashworth/lein-difftest