python tests: use py shebang instead of $(PYTHON)

I'm going to make the external tests manually launchable. For that I first had
an error message if $(PYTHON) was empty. But Pau says I should just use shebang
instead and ignore the autoconf python stuff, since that often fails anyway.

Change-Id: Ie35dd78c42577109a6a3143221a9769e47d361a5
diff --git a/tests/smpp_test_runner.py b/tests/smpp_test_runner.py
old mode 100644
new mode 100755
index 7a3a342..f6567d9
--- a/tests/smpp_test_runner.py
+++ b/tests/smpp_test_runner.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # (C) 2014 by Holger Hans Peter Freyther
 # based on vty_test_runner.py: