build: rebuild test addons conditionally

Before this commit, make test-addons and the targets that depend on it (test-ci, test-all) would always do a full rebuild of the files in test/addons and the addons scraped from doc/api/addons.markdown.

This commit introduces a proper dependency chain so that files are only rebuilt when changed, shaving off about 10-20 seconds from each run.

R=@chrisdickinson