Improve root cause tests to also verify JSON output

[john-h-kastner]

The root cause issue fixed in PR #666 was only visible when examining the root cause JSON output. Since the JSON output is parsed by 3c-wrap root_cause it is probably more important that it is correct than the clang diagnostics root causes. Our root cause tests do not currently look at the JSON output, so any bugs that manifest there but not in the diagnostics will not be detected.

Previous discussion: #666 (comment)