support/testing: use TestPythonPackageBase for python-incremental

Move the test script to be run on the target from inline in the test
case to 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>

support/testing/tests/package/sample_python_incremental.py

@@ -0,0 +1,3 @@
+import incremental
+v = incremental.Version("package", 1, 2, 3, release_candidate=4)
+assert(str(v) == "[package, version 1.2.3rc4]")