[lib] Write some tests for text-utils:pluralize(...)

Summary:
Suggested by @palys-swm:

Also, this is a great function to have some tests.

https://phab.comm.dev/D4317#121940

Took like a minute. Not sure if it helps much, but it doesn't hurt?

Test Plan: Ran tests

Reviewers: def-au1t, palys-swm, yayabosh, ashoat

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian, palys-swm

Differential Revision: https://phab.comm.dev/D4327