DSLReports had the first bufferbloat test I'm aware of, but it's broken now. Another example is https://www.waveform.com/tools/bufferbloat
Ideally this problem should be fixed by ISPs and modem manufacturers, but individuals can use OpenWrt with luci-app-sqm, at least when the connection has a predictable speed limit.
An option to choose between IPv4/IPv6 (or test both) would probably be useful.