Fix tor-browser tbb-tests

With current rebased tor-browser ESR68 branch I can only run tbb-tests (with run-tbb-tests script) when pref("network.file.path_blacklist", "/net") is removed and pref("extensions.torbutton.use_nontor_proxy", true); is set, apart from disabling tor-launcher. The second pref disables the domain isolator, which makes sense since it expects SOCKS5 proxies, but mochitests override that. For the other pref, not sure why network.file.path_blacklist needs to be unset (at least for Linux).

We could put these prefs in testing/marionette/prefs/marionette.js so that tests can be run (unless there is a simpler way to get the tests tor run that I'm missing).