Have lints for as much as possible

Originally posted as https://github.com/w3c/wpt-tools/issues/154 by @gsnedders on 31 Jan 2017, 07:24 UTC:

@patrickkettner was correctly pointing out that we should, ideally, have lints for almost everything we have rules about. Some things we're missing:

• reftests shouldn't have a canvas larger than the standard viewport
• generate_tests shouldn't be used
• linked resources should exist (we might need some way to have inline whitelisting for this, because I don't think the global whitelist would scale for <img src=non-existant-file> and similar?)
• files should either declare their encoding or be ASCII
• files should be valid (or, at the very least, contain no parse errors)
• manual tests if they use testharness.js should have an explicit timeout
• vendor prefixes aren't used
• complex assertion in assert_true
• setTimeout used (esp. with short time)
• .worker.js tests should import testharness.js.
• .any.js tests should not import testharness.js.
• catch basic cases of external URLs

There was also the discussion about whether we should require more when it comes to code-style, though that's likely something for the mailing list.

No files, tests, or entry points are named. Start by reviewing the existing linting system and the listed missing checks, then clarify which rules are in scope. Done requires an agreed scope and lint coverage for the selected rules.