[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