Add coverage for untested code paths and features; rm test noise

[botandrose]

A lot of the tricky code around head handling and value property handling was untested, so this PR brings our coverage way up, giving me much more confidence to try to add twoPass without regressions. Now we're only missing coverage for very minor codepaths.

I've also gone ahead and removed a lot of noise from the test output. This removal came in two flavors:

1. Most of the tests in test/core had extra console logs diffing the actual and expected output. This seems unnecessary given that the test itself performs and displays this diff, and also noticing that many of them were out of date and no longer reflected the actual intent of the test. Delete!
2. Other tests were dumping info that might be useful for debugging the test if something was wrong, but not useful while the tests are passing. I've commented these lines out and given them a header of // debugging output.

npm test is nice and pretty now!