tests: Add testprocess.wait_for_quit.

This was in conftest.py before, but we might want to use it outside of that as
well.

tests/integration/features/conftest.py

@@ -465,6 +465,4 @@ def clipboard_contains_multiline(qtbot, qapp, content):
 
 @bdd.then("qutebrowser should quit")
 def should_quit(qtbot, quteproc):
-    quteproc.exit_expected = True
-    with qtbot.waitSignal(quteproc.proc.finished, timeout=15000):
-        pass
+    quteproc.wait_for_quit()

tests/integration/testprocess.py

@@ -417,3 +417,9 @@ class Process(QObject):
             return
         else:
             raise BlacklistedMessageError(line)
+
+    def wait_for_quit(self):
+        """Wait until the process has quit."""
+        self.exit_expected = True
+        with self._wait_signal(self.proc.finished, timeout=15000):
+            pass