Use feature instead of exporting the crate for testing with hidden doc

Currently, iceoryx exposes an API for testing purpose and hides its docs. I think a more reasonable approach is define a feature called **iox2-test** for it so we can ensure it won't be involved inside users code except the explicit feature setting up. A testing feature is needed because the default `test` doesn't support cross package usage.

https://github.com/eclipse-iceoryx/iceoryx2/blob/558c727395f50793f4e341dae73dd7315c4f6a4f/iceoryx2-bb/elementary/src/bump_allocator.rs#L17-L20

Start with iceoryx2-bb/elementary/src/bump_allocator.rs at lines 17-20 and inspect how the testing API is currently exported and hidden from documentation. Trace the crate feature configuration and cross-package test setup; done means the API is available only when the explicit iox2-test feature is enabled and cross-package tests can use it.

Written by the indexing model from the issue text.