kunit: tool: allow generating test results in JSON

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#post


Signed-off-by: Heidi Fahim <heidifahim@google.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>