Stabilize ssl_strict test

tests/end2end/features/test_prompts_bdd.py

@@ -17,6 +17,8 @@
 # You should have received a copy of the GNU General Public License
 # along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
 
+import time
+
 import pytest_bdd as bdd
 bdd.scenarios('prompts.feature')
 
@@ -50,6 +52,7 @@ def no_prompt_shown(quteproc):
 def ssl_error_page(request, quteproc):
     if request.config.webengine and qtutils.version_check('5.9'):
         quteproc.wait_for(message="Certificate error: *")
+        time.sleep(0.5)  # Wait for error page to appear
         content = quteproc.get_content().strip()
         assert "ERR_INSECURE_RESPONSE" in content
     else: