tests: make killdaemons.py use DAEMON_PIDS by default

tests/killdaemons.py

@@ -87,5 +87,9 b' def killdaemons(pidfile, tryhard=True, r'
         pass
 
 if __name__ == '__main__':
-    path, = sys.argv[1:]
+    if len(sys.argv) > 1:
+        path, = sys.argv[1:]
+    else:
+        path = os.environ["DAEMON_PIDS"]
+
     killdaemons(path)