I don't know why you would use an extension for this since the behaviour is already built-in to chrome. Click the padlock next to the url -> site settings -> javascript off
I'm not sure if you can take that testmy.net source for granted. It mis-reported my "internet" speed by about 30Mbps. I get around 50Mbps on fast.com and speed.cloudflare.com - those two speedtests are enough for me to consider it "internet" speed.