tests: Use /usr/bin/env python3

* Use the env variable to $PATH is searched instead of hardcoded
* Enables a local python build to take priority over system python build
  as commonly used by tools like virtualenv.

test cases/common/56 custom target/my_compiler.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 

test cases/common/57 custom target chain/my_compiler.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 

test cases/common/57 custom target chain/my_compiler2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 

test cases/common/59 object generator/obj_generator.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 # Mimic a binary that generates an object file (e.g. windres).