Provide integration tests for Import/Export Members Mapper

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Why are we doing this work

During the discussion from !71867 (merged) we found out that specs are not failing if we remove very important property from import_export.yml file for users: id:

I wonder why user id was included in this list to begin with since we filter out all occurrences of id 🤔 Are we certain it's not used anywhere?

I checked members mapper and its actually used there https://gitlab.com/gitlab-org/gitlab/blob/ae74ea54daa39fd2c5941a8b4da8daf14fd82abc/lib/gitlab/import_export/members_mapper.rb#L21-L21 We need this id in order to build a map of member user ids old_user_id -> new_user_id in order for the member mapping to work correctly.

The reason it works is because members mapper runs as a one of the first steps in the import process, before cleaner/permitter filters it out https://gitlab.com/gitlab-org/gitlab/blob/ae74ea54daa39fd2c5941a8b4da8daf14fd82abc/lib/gitlab/import_export/project/tree_restorer.rb#L31-L31

We need to add additional specs to make sure this is tested and that the ID will not be removed by accident from that file.

Implementation plan

• backend add additional spec that will export a project with few user members and import it as a new project and verify if users are properly mapped,