buildroot

testing/tests/package/test_python: do not hardcode interpreter name

In order to be able to leverage the same test code for testing
different python interpreters (or wrappers around CPython) allow child
classes of TestPythonBase to override the name of the executable used
to run tests.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>