fix: event entrypoint incorrectly pointed to request type registry

[SilvyPuzzlewell]

Description

During initialization of invenio_requests request types, event types and entity references are loaded from entrypoints to the respective registries. There was a copypaste error in init_registry that caused that event type entrypoints were loaded incorrectly into request type registry. This patch fixes it by using the correct registry.

Tests were added to show that all entrypoints are loaded correctly.

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