Fix JSON output of libtest

[oriongonza]

Right now the JSON output of libtest is a bit unusable. It outputs all of the json messages to stdout where they get mixed with the rest of the stdout.

It would be nice to be able to output it to a file (it shouldn't particularly hard to do since the OutputFormatter only requires Write) and also to be able to customize what the json formatter outputs.

@rustbot tag +T-bootrstap