Test scripts should not hard-wire python executable

[Prof Garth Wells]

To test with py2 and py3, the tests scripts should not hard-wire the Python interpreter.

[Martin Sandve Alnæs]

Use environment variable if set in test bash scripts to fix issue 18.

→ <<cset 8e1345556cb7>>

[Martin Sandve Alnæs]

pipelines already use py.test-3, now the shell scripts use $PYTHON if available. Considering this solved, otherwise more details needed.