[functional testing] Determine how to handle TB initial startup window

[redshiftzero]

When Tor Browser starts for the first time, a window pops up asking the user some questions (e.g. should Tor use a bridge, etc.). We need to programmatically click buttons on this window or suppress it in order to use Tor Browser for functional testing.

Epic: #3659

[msheiny]

seems like those settings are stored in the user profile which from an unpacked tar is TorBrowser/Data/Browser/profile.default. from there I see a bunch of settings configured in prefs.js this one in particular (i'm sure there are others) is of note extensions.torlauncher.prompt_at_startup --> false

[eloquence]

Done in feature branch via #3628.