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/enb_amarisoft.py b/src/osmo_gsm_tester/obj/enb_amarisoft.py
index 60985f4..b7ede3b 100644
--- a/src/osmo_gsm_tester/obj/enb_amarisoft.py
+++ b/src/osmo_gsm_tester/obj/enb_amarisoft.py
@@ -130,6 +130,10 @@
         self.testenv.remember_to_stop(self.process)
         self.process.launch()
 
+    def stop(self):
+        # Not implemented
+        pass
+
     def gen_conf_file(self, path, filename, values):
         self.dbg('AmarisoftENB ' + filename + ':\n' + pprint.pformat(values))
         with open(path, 'w') as f: