bdd: Stabilize "Closing a JS window twice" test.

After pressing the button to open a window, we have to wait until it's loaded
before continuing, otherwise the test is flaky:
http://www.qutebrowser.org/testresults/osx/1295.html

We can't simply wait with "wait until about:blank is loaded" as that page is
already loaded earlier.

tests/integration/features/javascript.feature

@@ -9,6 +9,7 @@ Feature: Javascript stuff
         And I open data/javascript/issue906.html in a new tab
         And I run :hint
         And I run :follow-hint a
+        And I wait for "Changing title for idx 2 to 'about:blank'" in the log
         And I run :tab-focus 2
         And I run :hint
         And I run :follow-hint s