Json test output formatter should represents "test_count" as a json string instead of a number

[cwndrws]

The output of a test suite starting when using the json formatter represents "test_count" as a json string, like so:

{ "type": "suite", "event": "started", "test_count": "1" }

I would expect it to be a number like this:

{ "type": "suite", "event": "started", "test_count": 1 }

[cwndrws]

It also seems like "filtered_out" on run finish is also unexectedly a string.