ci: Re-enable python subtests in qtest migration suite

The migration compatibility tests have been running with the PYTHON
variable unset to avoid running a broken test. The faulty test has
since been removed, so we can enable the python tests once again.

Aside from the broken test, only one other test uses python and I have
been running it locally ever since, so this commit should not expose
any new bug.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250513133353.23022-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

.gitlab-ci.d/buildtest.yml

@@ -227,14 +227,6 @@ build-previous-qemu:
     # testing an old QEMU against new features/tests that it is not
     # compatible with.
     - cd build-previous
-    # Don't allow python-based tests to run. The
-    # vmstate-checker-script test has a race that causes it to fail
-    # sometimes. It cannot be fixed it because this job runs the test
-    # from the old QEMU version. The test will be removed on master,
-    # but this job will only see the change in the next release.
-    #
-    # TODO: remove this line after 9.2 release
-    - unset PYTHON
     # old to new
     - QTEST_QEMU_BINARY_SRC=./qemu-system-${TARGET}
           QTEST_QEMU_BINARY=../build/qemu-system-${TARGET} ./tests/qtest/migration-test