The Unsafe Browser tests sometimes fail due to too fast keyboard interaction

In this code:

Then /^I cannot configure the Unsafe Browser to use any local proxies$/ do
  next if @skip_steps_while_restoring_background
  @screen.wait_and_click("UnsafeBrowserWindow.png", 10)
  sleep 0.5
  # First we open the proxy settings page to prepare it with the
  # correct open tabs for the loop below.
  @screen.type("e", Sikuli::KeyModifier.ALT)
  @screen.type("n")
  @screen.wait('UnsafeBrowserPreferences.png', 10)

I’ve seen the “n” typing fail, most likely because the menu’s keybindings having not been set up fast enough for it to trigger any action. I suggest we instead wait_and_click on the Preferences menu entry.

Feature Branch: feature/tor-browser-bundle

Parent Task: #8539 (closed)

Original created by @intrigeri on 8009 (Redmine)