JSON strings appearing in phpunit test output

[brettshumaker]

Description

There's a particular test that causes a JSON string to be output while running PHPUnit. I think this has been happening for a while but I decided to investigate it today. I've tracked it down to these two switch conditions in test_maybe_handle_onboarding_connect_from_known_from.

Would it be fine to wrap the $wcpay_account->maybe_handle_onboarding(); at the bottom of that test in ob_start() and ob_end_clean() to clean up the PHPUnit output?

cc @dmallory42 @vladolaru as the two folks I see who contributed to that section.