Add :sort command

testing/runner.py

@@ -143,3 +143,14 @@ def and_exit(h):
     if ' *' in h.get_screen_line(0):
         h.press('n')
     h.await_exit()
+
+
+def trigger_command_mode(h):
+    # in order to enter a steady state, trigger an unknown key first and then
+    # press escape to open the command mode.  this is necessary as `Escape` is
+    # the start of "escape sequences" and sending characters too quickly will
+    # be interpreted as a single keypress
+    h.press('^J')
+    h.await_text('unknown key')
+    h.press('Escape')
+    h.await_text_missing('unknown key')