mhtml: reduce test flakiness

Waiting for "All assets downloaded" may lead to race conditions when
the output file is read before qute wrote it, so the test fails.

qutebrowser/browser/mhtml.py

@@ -436,6 +436,7 @@ class _Downloader():
                 self.writer.write_to(file_output)
             finally:
                 file_output.close()
+        log.downloads.debug("File successfully written.")
         message.info(self._win_id, "Page saved as {}".format(self.dest))
 
     def collect_zombies(self):

tests/integration/test_mhtml_e2e.py

@@ -86,7 +86,7 @@ def test_mhtml(test_name, download_dir, quteproc, httpbin):
     quteproc.send_cmd(':download --mhtml --dest "{}"'.format(download_dest))
     quteproc.wait_for(category='downloads', module='mhtml',
                       function='finish_file',
-                      message='All assets downloaded, ready to finish off!')
+                      message='File successfully written.')
 
     expected_file = os.path.join(test_dir, '{}.mht'.format(test_name))
     download_dir.compare_mhtml(expected_file)