Wait for quteproc shutdown in across-restarts prompt test

This is failing with:

    ERROR tests/end2end/test_invocations.py::test_permission_prompt_across_restart - PermissionError: [WinError 5] Access is denied: 'C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\tmpytep6gj0\\cache\\webengine\\Cache\\Cache_Data\\data_0'

In pytest teardown, while trying to clean up a temp dir, probably the
basedir. The test above waits for shutdown at the end of the test, maybe
that's what's needed here.

Otherwise maybe just `@pytest.mark.skipif(utils.is_windows)` 🤷

tests/end2end/test_invocations.py

@@ -662,6 +662,9 @@ def test_permission_prompt_across_restart(quteproc_new, request, short_tmpdir):
     # We should be re-prompted in the new instance
     notification_prompt('no')
 
+    quteproc_new.send_cmd(':quit')
+    quteproc_new.wait_for_quit()
+
 
 # The 'colors' dictionaries in the parametrize decorator below have (QtWebEngine
 # version, CPU architecture) as keys. Either of those (or both) can be None to