python3: use env python invocation in tools

In order to facilitate testing, but not necessarily as an example of
good practice, I am changing the invocation of the test tools to use
`/usr/bin/env python`, so that we can control which python (2 vs 3)
gets invoked for the test. On the buildbots, I plan to add an optional
`ln -s /usr/bin/python3 /usr/local/bin/python` on systems that have
python3-bcc package built. This way, we get more test coverage. Having a
cmake mechanism to enable both python2 and python3 testing could be a
further enhancement.

tests/python/test_stackid.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright (c) PLUMgrid, Inc.
 # Licensed under the Apache License, Version 2.0 (the "License")
 

tests/python/test_usdt.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # USAGE: test_usdt.py
 #

tests/python/test_usdt2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # USAGE: test_usdt2.py
 #

tests/python/test_usdt3.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # USAGE: test_usdt3.py
 #

tests/python/test_utils.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright (c) Catalysts GmbH
 # Licensed under the Apache License, Version 2.0 (the "License")