tests: Wait until page is dumped for header check.

If we don't do this, it's possible for the test process to delete the temporary
directory before qutebrowser actually writes the file.

qutebrowser/browser/commands.py

@@ -1242,6 +1242,7 @@ class CommandDispatcher:
             data = mainframe.toHtml()
         with open(dest, 'w', encoding='utf-8') as f:
             f.write(data)
+        message.info(self._win_id, "Dumped page to {}.".format(dest))
 
     @cmdutils.register(instance='command-dispatcher', name='help',
                        completion=[usertypes.Completion.helptopic],

tests/integration/quteprocess.py

@@ -268,6 +268,8 @@ class QuteProc(testprocess.Process):
         with tempfile.TemporaryDirectory() as tmpdir:
             path = os.path.join(tmpdir, 'page')
             self.send_cmd(':debug-dump-page --plain "{}"'.format(path))
+            self.wait_for(category='message', loglevel=logging.INFO,
+                          message='Dumped page to {}.'.format(path))
 
             with open(path, 'r', encoding='utf-8') as f:
                 return f.read()