Auto merge of #33020 - nikomatsakis:compiletest-json, r=alexcrichton

port compiletest to use JSON output

This uncovered a lot of bugs in compiletest and also some shortcomings
of our existing JSON output. We had to add information to the JSON
output, such as suggested text and macro backtraces. We also had to fix
various bugs in the existing tests.

Joint work with @jonathandturner.

r? @alexcrichton