Add --json option to save collector payload to JSON

[nprizal]

We’re integrating bktec with pytest, which requires reading the output from pytest. While pytest has native JUnit XML, it lacks certain information required by bktec. Since bktec relies on this collector, it’s more convenient to save the JSON output from this collector, so bktec can read it.

To save json payload to a file, you need to add --json=path option to the pytest command. For example

pytest --json=result.json

[zhming0]

Looks good to me.

[zhming0]

[nit] the comment looks like a better method name

Suggested change

	def save_payload(self, path):
	""" Save payload into a json file """
	def save_payload_as_json(self, path):
	""" Save payload into a json file """

[nprizal]

@zhming0 I made some changes to this PR to only save test data.
Instead of

{
  "format": "json",
  "run_env": {},
  "data": [{"id": "test-1", ...}, {"id": "test-2", ...}]
}

we save

[{"id": "test-1", ...}, {"id": "test-2", ...}]

Let me know what you think

[zhming0]

Looks good with the format change.

Overall, I do have a bit concern around the fact that this looks like a "one-way door"-ish. But this is bound to be a problem with bktec and collectors having independent lifecycle, so I don't see harm in getting this change out.