Switch tests to shift-parser-expectations

[bakkot]

This PR:

• Modifies the existing test262-parser-tests test to additionally check ASTs and metadata against those in the just-published shift-parser-expectations package
• Modifies the test harness to assert tests in this project are not duplicated upstream in shift-parser-expectations
• Adds a script to automatically remove tests in this project which are so duplicated
• Removes those tests (using that script and eslint --fix, so this adds a bit of whitespace)
• Modifies the eslintrc for tests to allow unused variables, since many of the files are now effectively empty but we don't want to be constantly adding and removing helpers.

[bakkot]

Also now removes some apparently dead code in early-error-state.js. I don't know what that's about - coverage complains it's never hit.