Streamlined `wp_send_json_success` handling.

Now testing AJAX is about as simple as it can get.

New method available on the WP_Ajax_UnitTestCase class:

/**
	 * Capture and return the "data" key from any call to
	 * `wp_send_json_success` or `wp_send_json_error`.
	 *
	 * Automatically handles:
	 * 1. Reset $this->_last_response
	 * 2. Catch the exception
	 * 3. Return the result that would be sent as `data`.
	 *
	 * @author Mat Lipe
	 *
	 * @since 1.7.0
	 *
	 * @param callable $callable
	 *
	 * @return mixed
	 */
	protected function _getJsonResult( callable $callable ) {
		unset( $this->_last_response );
		try {
			$this->_handleAjaxCustom( $callable );
		} catch ( \Exception $exception ) {
			return json_decode( $this->_last_response )->data;
		}
	}

No more boilerplate or try/catch need to test typical results.

Must be using standard wp_send_json_success or wp_send_json_error functions to use this. Or at least by following the same wp_die and data key patterns.