Is it possible to designate an alternative wifi connection test site?
Yes. Since Android 7.1 the method is a little different (not gonna mention the old method here) - fire up adb shell then use the command settings put global captive_portal_https_url **site domain here**.

In the past g.cn (Google's remaining Chinese domain) worked alright, but since Nougat for better reliability I'm using one hosted by V2EX: captive.v2ex.co/generate_204, so the full command becomes settings put global captive_portal_https_url https://captive.v2ex.co/generate_204.