[test] Use python3 for test wrappers

test/freeflow/navierstokes/sincos/convergencetest.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from math import *
 import subprocess

test/multidomain/boundary/stokesdarcy/1p_1p/convergencetest/convergencetest.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from math import *
 import subprocess

test/multidomain/boundary/stokesdarcy/1p_1p/convergencetest/manufactured_solution.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 This script determines the source terms needed for analytical solutions for coupled Stokes/Darcy problems.

test/multidomain/facet/1p_1p/analytical/convergencetest.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from math import *
 import os.path

test/porousmediumflow/1p/implicit/network1d3d/convergencetest.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from math import *
 import subprocess