environment variable to skip TorButton control port verification

Tor Button now does its own Tor ControlPort verification.

TBB now performs its own control port verification. We simply check that the socks port Tor says its on is the one Firefox is configured to use.

If that test fails, the default homepage about:tor will show:

Something Went Wrong!
Tor is not working in this browser.

This is problematic in transparent / isolating proxy environments, that do not wish to allow ControlPort access for security reasons (getinfo address and other dangerous ControlPort commands).

The test is implemented in torbrowser.js, in function torbutton_local_tor_check().

[...]
  const kCmdArg = "net/listeners/socks";
[...]
  let resp = torbutton_send_ctrl_cmd("GETINFO " + kCmdArg + "\r\n");
[...]

(The environment variable TOR_SKIP_LAUNCH=1 does not prevent this. [No such variable in Tor Button's source code.])

It would be nice if there was an environment variable such as TOR_SKIP_CONTROLPORTTEST (or so), that would skip this test.

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