Add tests for knit_print output

[mitchelloharawild]

#108 could have been prevented if knit_print tests captured this breaking change.

Adding test examples checking for knit_print output should provide reasonably detailed tests for brief_entries(), detailed_entries() and bibliography_entries() outputs.

[johnaeanderson]

#108 still isn't fixed for me after grabbing the latest version of vitae. I'm still getting

Error: No common type for year and year ... I never had these issues with the previous versions. I expect that tibble has suddenly changed to only allow one class.

[mitchelloharawild]

Yes, this would also be due to changes in tibble. Could you post another issue containing a minimally reproducible example (https://www.tidyverse.org/help/) of this issue?