Fix spec violations in delete_rotation_modal_spec.js

What does this MR do and why?

• get rid of mocking data()
• use preferred way for mocking Apollo
• unify createComponent and duplicated assertions
• general refactor

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Checkout the branch locally and run: yarn jest ee/spec/frontend/oncall_schedule/rotations/components/delete_rotation_modal_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #413319 (closed)