tests: Start testing migration scripts
This adds the tools and the last 2 scripts to demonstrate how to start testing the migration scripts.
This is a preparatory work for testing the keycloak migration.
Note that a new arborescence tree swh/deposit/tests_migration
got added to
avoid conflict with the other tests. The new folder path got chosen to avoid moving
too much files.
Related to #2858 (closed)
Migrated from D5228 (view on Phabricator)