Use `go test -json` in updateFailing, detect baseline errors

[jakebailey]

This does two things:

• Uses go test -json to stream the output in a parseable format.
• With the parsed output, read each failing test's output to see if all of its failures are due to missing baselines.

We have no tests where this matters (I or others had fixed them previously), but I have a change where it does matter in the future.

[jakebailey]

I need to think about this harder; I have to go manually merge this into one of my branches to get it to test so it's annoying to figure out.

[jakebailey]

No, this does seem to work, I just merged the two PRs together wrong.