refactor(bbr): move test helpers into plugins/test

[yehuditkerido]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Move pkg/bbr/test/ into pkg/bbr/plugins/test/ so test helpers live alongside the plugin code they support. Rename helpers.go to base_model_plugin.go to reflect its content, and drop the bbrtest import alias in favor of the directory-derived test package name.

Does this PR introduce a user-facing change?:
None

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[k8s-ci-robot]

Hi @yehuditkerido. Thanks for your PR.

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[nirrozenbaum]

/ok-to-test

[nirrozenbaum]

/lgtm
/approve

thanks 🙏

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nirrozenbaum, yehuditkerido

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[nirrozenbaum]

@yehuditkerido I think it's required to rebase.

[nirrozenbaum]

/lgtm