monkey-patch console.log and console.warn for all tests

[Rich-Harris]

At present we do one of two things with logs in tests:

• monkey-patch console.log and console.warn on a per-test basis. this involves lots of repetitious ugly code
• have a separate log.js file that exports a log array and do log.push instead of console.log. this is also repetitious (and requires a before_test or after_test hook to reset between different test modes), and is particularly annoying because in order to quickly try something out in the playground you need to rewrite the test if you want to actually see the logs

This PR makes things much simpler: we can access logs and warnings directly inside the test function.

If something gets logged before the test runs, and the test doesn't use logs, then the logs get stashed away, and replayed when either console.log is first called inside the test or, if that doesn't happen, after the test has run, meaning that temporary logs will always be visible and in the correct order.

Draft because I haven't yet converted all the tests that use the old way of doing things

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