Test (most of) the rest of WaypointMarker.

There isn't any UI observable behavior of the dragend of the waypoint,
but we can test that the backend was called. The only uncovered parts of
that component are now error paths, but the error handling in that
component is to just ignore errors, so there's also nothing to test
there.

client/src/api/backend.ts

@@ -4,7 +4,10 @@ const backendAddr =
   new URL(window.location.toString()).searchParams.get("server") ??
   "[::1]:16880";
 
-export const HTTP_URL = `http://${backendAddr}/`;
+// MSW can't handle IPv6 URLs...
+// https://github.com/mswjs/msw/issues/1388
+export const HTTP_URL =
+  process.env.NODE_ENV === "test" ? "" : `http://${backendAddr}/`;
 
 export const backend = axios.create({
   baseURL: HTTP_URL,