[EPIC] Testing Improvements

### Certification

- [x] I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

### User Story

As a developer, I want standardized test practices across our codebase so that our tests have good DevX with clear guidelines, are more maintainable, resilient to UI changes, and aligned with Cypress best practices.

Start by reviewing the issue's planning scope for standardized testing practices and its stated Cypress best-practices goals; no files, tests, or entry points are named. Define the affected codebase areas and acceptance criteria before changing anything, with completion requiring maintainable, resilient tests and clear shared guidelines.

Written by the indexing model from the issue text.