ci: migration: Don't run python tests in the compat job

The vmstate-checker-script test has a bug that makes it flaky. It was
also committed by mistake and will be removed.

Since the migration-compat job takes the tests from the build-previous
job instead of the current HEAD, neither a fix or a removal of the
test will take effect for this release.

Disable the faulty/undesirable test by taking advantage that it only
runs if the PYTHON environment variable is set. This also disables the
analyze-migration-script test, but this is fine because that test
doesn't have migration compatibility implications.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20240905185445.8179-1-farosas@suse.de
[peterx: Added a TODO to remove the line after 9.2 release, per thuth]
Signed-off-by: Peter Xu <peterx@redhat.com>

.gitlab-ci.d/buildtest.yml

@@ -212,6 +212,14 @@ 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