Support json format for --list

[dzfranklin]

I'd like to be able to get a json-formatted list of the tests and benches. This isn't a huge deal, because the output is pretty simple to parse with regex.

Expected:

> test_binary -Z unstable-options --list --format json
[{name: "foo", type: "test"}]

Actual:

> test_binary -Z unstable-options --list --format json
foo: test

1 test, 0 benchmarks