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

Summary: There's only three functions in text-utils so might as well crank out some unit tests for all of them.

Test Plan: Ran tests

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4328