Add a --json flag, which when specified generates JSON formatted test
results conforming to the KernelCI API test_group spec[1]. The user can
use the new flag to specify a filename to print the json formatted
results to.

Link[1]: https://api.kernelci.org/schema-test-group.html#postSigned-off-by: NHeidi Fahim <heidifahim@google.com>
Signed-off-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>