Add CI instructions to test generated HTMLs

Merged Phu Nguyen requested to merge work/htmltest into master
  • Can test not only external URLs, but also internal ones, as well as alt texts and other stuff. Individual HTML tags can be ignored
  • Run in .post stage (for now) since we don't need it to halt anything with its result
  • .htmltest/refcache.json file can be used by any job running htmltest, so it's cached with the htmltest-refcache key
  • Test results are saved as an artifact
Edited by Phu Nguyen

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