functional_tests: ensure mining stops on error in mining test

this prevents messing up any subsequent test too

tests/functional_tests/mining.py

@@ -170,5 +170,15 @@ class MiningTest():
             assert res.hash == block_hash
 
 
+class Guard:
+    def __enter__(self):
+        pass
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        daemon = Daemon()
+        try: daemon.stop_mining()
+        except: pass
+
 if __name__ == '__main__':
+    with Guard() as guard:
         MiningTest().run_test()