Fixes flaky spec in`spec/frontend/ide/components/merge_requests/list_spec.js`

Fixes flaky spec in list_spec.

Failure: Ran 100000 timers, and there are still more! Assuming we've hit an infinite recursion and bailing out...
Cause: jest.runAllTimers();
Solution: Mock out debounce with a global helper function in spec/frontend/__mocks__/debounce.js

#212532 (closed)

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This MR was created during this pair programming session