IntervalCollection: Break apart large test (#24890)

This pull request refactors and enhances the test suite for
`SharedInterval` in `sharedInterval.spec.ts`. The changes primarily
focus on improving test organization, reusability, and coverage of
multi-client scenarios. Key updates include introducing a setup function
for conflicting operations, restructuring tests into individual `it`
blocks, and ensuring synchronization and validation of interval
operations across clients.

Author: anthony-murphy