I use speedtest.net but not for this purpose, since the results are useless for streaming. If your connection is completely borked-- your computer misconfigured, your router failing, a local cable is defective, then a generic speedtest will tell you that.
For problems that exist beyond your telco's last mile, speedtest results, regardless of which site you use, tell you nothing. ISPs deliberately optimize those connections to give fast results, and the sites themselves automatically choose the server closest to you in order to give you the fastest test result possible.
TwitchTest actually tests against Twitch ingest servers, and gives you useful results. There's no equivalent for YouTube or mixer, although mixer.com does have its own speedtest function, so I presume that's more valid than a generic test but perhaps not as valid as TwitchTest.
In fact, even if you don't use Twitch, use TwitchTest; it will show the consequences of things like your ISP doing traffic shaping or connection resets on you because of large upstream traffic amounts. What it wouldn't tell you is that if there's a particular problem, right now, between your ISP's edge router and YouTube ingest servers, that you should complain about to them.