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I believe Nintendo uses http://conntest.nintendowifi.net/ for their consoles.

Edit: an update (3.0.0) to the Switch changed it to http://ctest.cdn.nintendo.net/ but I can't seem to access it from a web browser.

For ctest to work, it's necessary to start your User-Agent string with "NX NIFM/" (the console itself sends "NX NIFM/00" as an User-Agent string).