Update Thunderbird tests for Chinese
As reported on #18416 (comment 172149), on the testing branch and branches based on it, in "Tails is localized for every tier-1 language", "the screen keyboard works in Thunderbird" fails in Chinese: the failure screenshot does show the screen keyboard, but Dogtail fails to find Thunderbird via "I start Thunderbird".
I suspect it's due to localization changes coming with a recent Thunderbird security update. I also suspect this affects stable
as well (but we don't run this @fragile
test there).