Add Tests for use with custom entity manager

[Chris53897]

There are some problems that users reported, which occur only with the use of a custom entity manager.
If someone has some time, to write test for ot, that would be great.
#76 (comment)

        entity_managers:
            default:
                connection: dynamic
                naming_strategy: doctrine.orm.naming_strategy.underscore
           ...
            main:
                connection: main
                naming_strategy: doctrine.orm.naming_strategy.underscore
                mappings:
                    main:
                        is_bundle: false
                        dir: "%kernel.project_dir%/src/Entity/main"
                        prefix: 'App\Entity\main'
                    DukecityCommandSchedulerBundle: ~