Abstract registering signal handler in the InputHandler constructor

This small change is done to simplify testing of the code.

Related: rhbz#2134888

(cherry picked from commit f0d8966)

Author: jkonecny12

simpleline/input/input_handler.py

@@ -52,6 +52,9 @@ def __init__(self, callback=None, source=None):
         self._skip_concurrency_check = False
         self._source = source
 
+        self._register_input_ready_signal()
+
+    def _register_input_ready_signal(self):
         App.get_event_loop().register_signal_handler(InputReadySignal,
                                                      self._input_received_handler)