Test suite: add some retry_tor magics to the WebM in Tor Browser scenario.

I've tried bumping the number of vCPUs to 4, giving the VM under test more RAM,
and it's still very unreliable for me. The Journal says:

  Jul 06 08:54:49 amnesia tor-browser.desktop[8445]: [Child 8706, MediaPlayback #1] WARNING: Decoder=72da91792200 state=DECODING_METADATA Decode metadata failed, shutting down decoder: file /var/tmp/build/firefox-a0efd2fcd6e9/dom/media/MediaDecoderStateMachine.cpp, line 379
  Jul 06 08:54:49 amnesia tor-browser.desktop[8445]: [Child 8706, MediaPlayback #1] WARNING: Decoder=72da91792200 Decode error: NS_ERROR_DOM_MEDIA_METADATA_ERR (0x806e0006): file /var/tmp/build/firefox-a0efd2fcd6e9/dom/media/MediaDecoderStateMachine.cpp, line 3445

The previous step passes fine so it's not a matter of connecting to our website,
so I have some doubt this change will help for this specific problem, but well,
it's a good thing anyway.

features/step_definitions/browser.rb

@@ -218,3 +218,20 @@ Then /^the Tor Browser shows the "([^"]+)" error$/ do |error|
   found = headers.any? { |heading| heading.text == error }
   raise "Could not find the '#{error}' error in the Tor Browser" unless found
 end
+
+Then /^I can watch a WebM video in Tor Browser$/ do
+  test_url = 'https://tails.boum.org/lib/test_suite/test.webm'
+  info = xul_application_info('Tor Browser')
+  open_webm_url = Proc.new do
+    step "I open the address \"#{test_url}\" in the Tor Browser"
+  end
+  recovery_on_failure = Proc.new do
+    @screen.type(Sikuli::Key.ESC)
+    @screen.waitVanish(info[:browser_reload_button_image], 3)
+    open_webm_url.call
+  end
+  open_webm_url.call
+  retry_tor(recovery_on_failure) do
+    @screen.wait("TorBrowserSampleRemoteWebMVideoFrame.png", 30)
+  end
+end

features/torified_browsing.feature

@@ -55,8 +55,7 @@ Feature: Browsing the web using the Tor Browser
     Given I have started Tails from DVD and logged in and the network is connected
     When I start the Tor Browser
     And the Tor Browser loads the startup page
-    And I open the address "https://tails.boum.org/lib/test_suite/test.webm" in the Tor Browser
-    Then I see "TorBrowserSampleRemoteWebMVideoFrame.png" after at most 180 seconds
+    Then I can watch a WebM video in Tor Browser
 
   #11592
   @fragile