Add shebang to pinvalidate.py

In the Standard Pin Names porting guide, pinvalidate.py is run
without prefixing the "python" command. But to make it possible,
a shebang needs to exist.

hal/tests/pinvalidate/pinvalidate.py

@@ -1,3 +1,5 @@
+#!/usr/bin/env python
+
 """
 Copyright (c) 2020 ARM Limited