For me, testing a browser is seeing how much unsollicited requests it does when running it for the first time. If it makes more than 10 unsollicited requests, it is not a privacy-respectful browser, as I call them.
The program that I use to test a browser is called Fiddler. It is a nice program that works as a proxy to see what and how many connections a program makes. It's just like MITMproxy. There's also Wireshark, but I couldn't make it work. (sad face)
You have to change a few options in Fiddler's settings to have the best capture as possible.
First, start Fiddler, go to Tools and then Telerik Fiddler Options. In the General ttab, check "Enable high-resolution timers" and uncheck "Notify me for updates on startup".
In the HTTPS tab, check "Capture HTTPS CONNECTs" if it's not already, and check "Decrypt HTTPS traffic". You can now click OK to save the settings and to close the window. You now have a configured Fiddler, ready to use!