testing/tests/package/test_python: refactor TestPythonBase

Convert TestPythonBase to a true base class that only provides code
implementing various tests without defining tests themselves in a
"discoverable" form.

To retain correct testing functionality, add TestPython2 derived class
that uses code from TestPythonBase to define actual runnable test.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Thomas: fix typo in commit log, update .gitlab-ci.yml, both pointed
by Ricardo.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 4e494ff..fe55763 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -226,7 +226,7 @@
 tests.fs.test_ubi.TestUbi: *runtime_test
 tests.fs.test_yaffs2.TestYaffs2: *runtime_test
 tests.package.test_dropbear.TestDropbear: *runtime_test
-tests.package.test_python.TestPythonBase: *runtime_test
+tests.package.test_python.TestPython2: *runtime_test
 tests.toolchain.test_external.TestExternalToolchainBuildrootMusl: *runtime_test
 tests.toolchain.test_external.TestExternalToolchainBuildrootuClibc: *runtime_test
 tests.toolchain.test_external.TestExternalToolchainCtngMusl: *runtime_test