enb: add abstract method stop()

implement as noop for Amarisoft eNB, srsENB will send q+Enter to stdin,
which is implemented in class srslte_common()

Change-Id: Ide606e1a6b523997215aa2fa39d4d56ae1f49181
diff --git a/src/osmo_gsm_tester/obj/srslte_common.py b/src/osmo_gsm_tester/obj/srslte_common.py
index 1d03a58..21001b7 100644
--- a/src/osmo_gsm_tester/obj/srslte_common.py
+++ b/src/osmo_gsm_tester/obj/srslte_common.py
@@ -36,6 +36,8 @@
             self.stop_sleep_time = 0
 
     def stop(self):
+        # Send q+Enter to stdin to self-terminate application
+        self.process.stdin_write('q\n')
         self.testenv.stop_process(self.process)
         self.sleep_after_stop()