Attempt to solve flaky editor tests.

These are passing locally but failing in travis. This fixes two possible
timing issues:

- Ensure the signals are set up befor the pidfile is written. The
  function that sends the signal waits for the pidfile to exist, so this
  ensures we don't miss a signal.
- Wait for the log message indicating that the editor file was read
  back, so the test doesn't run through before we get a chance to read
  from the editor.

tests/end2end/features/editor.feature

@@ -138,6 +138,7 @@ Feature: Opening external editors
         And I wait for "Entering mode KeyMode.insert (reason: clicking input)" in the log
         And I run :open-editor
         And I save without exiting the editor
+        And I wait for "Read back: foobar" in the log
         And I run :click-element id qute-button
         Then the javascript message "text: foobar" should be logged
 

tests/end2end/features/test_editor_bdd.py

@@ -90,11 +90,12 @@ def set_up_editor_wait(quteproc, tmpdir, text):
             if sig == signal.SIGUSR1:
                 sys.exit(0)
 
+        signal.signal(signal.SIGUSR1, handle)
+        signal.signal(signal.SIGUSR2, handle)
+
         with open(r'{pidfile}', 'w') as f:
             f.write(str(os.getpid()))
 
-        signal.signal(signal.SIGUSR1, handle)
-        signal.signal(signal.SIGUSR2, handle)
         time.sleep(100)
     """.format(pidfile=pidfile, text=text)))
     editor = json.dumps([sys.executable, str(script), '{}'])