[lib] Introduce tests for tMediaMessage[Photo/Video/Media]

Summary:
Introduced these in previous diff (D4990).

Just some basic tests mostly as a sanity check for myself since I have minimal experience with tcomb.

Test Plan: Run these tests (they're included in CI as well)

Reviewers: abosh, jacek, tomek, ashoat

Reviewed By: abosh, tomek

Differential Revision: https://phab.comm.dev/D4991