askrene: fix API breakage, add tests.

We cannot add new parameters in the middle, since we accept parameters by JSON
array as well as by dicts.  In fact, this broke tests, but due to unrelated
breakage in the GitHub "Automerge" functionality, it got applied as
556e38c838 ("askrene-bias-channel: bias call add
up.").

Also add tests, and a better Changelog line.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `askrene-bias-channel` now has a `relative` option to add, rather than replace, a channel bias.

plugins/askrene/askrene.c

@@ -1104,8 +1104,8 @@ static struct command_result *json_askrene_bias_channel(struct command *cmd,
 		   p_req("layer", param_known_layer, &layer),
 		   p_req("short_channel_id_dir", param_short_channel_id_dir, &scidd),
 		   p_req("bias", param_s8_hundred, &bias),
-		   p_opt_def("relative", param_bool, &relative, false),
 		   p_opt("description", param_string, &description),
+		   p_opt_def("relative", param_bool, &relative, false),
 		   NULL))
 		return command_param_failed();
 	plugin_log(cmd->plugin, LOG_TRACE, "%s called: %.*s", __func__,