feat(menu): add more tests for MatMenu component

Adds test coverage for:
- ARIA attribute inputs (aria-label, aria-labelledby, aria-describedby)
- Keyboard navigation with dynamically added/removed items
- `hasBackdrop` input property, including null/undefined cases
- Complex 3-level nested sub-menus (mouse & keyboard interactions)
- `overlapTrigger` with various x/y position combinations

Author: google-labs-jules[bot]