Add tests for `taxmap`

[zachary-foster]

This could take a while since there are a lot of functions with flexible input/output associated with taxmap. The list below includes the relatively complex functions that are not just wrappers for other functions:

• [x] all_names
• [x] names_used
• [x] get_data
• [x] obs
• [x] filter_taxa
• [x] filter_obs
• [x] select_obs
• [x] mutate_obs
• [x] transmute_obs
• [x] arrange_obs
• [x] arrange_taxa
• [x] sample_n_obs
• [x] sample_n_taxa