Adds a new "structure" PHPUnit test, "CodexMessageDefinitionTest.php".
This test exists to ensure that every message in the Codex message keys
file (produced by the library during build and updated for each Codex
release) is defined in the en.json and qqq.json message files for Codex
in MediaWiki.
Bug: T371330
Change-Id: I01f0888d13122d587bef17043eec46c32e8f534e
