support/testing: use TestPythonPackageBase for python-twisted

support/testing: use TestPythonPackageBase for python-twisted

Move the test script to be run on the target from inline in the test
case to a separate file.

Get the base defconfig fragment from the immediate parent class and not
directly from TestPythonBase because it is the correct way of doing
this. This way the base class TestPythonTwisted could even be placed in
a separate file.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Asaf Kahlon <asafka7@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>