CI: Add test-run caching for unified job retry optimization

Adds reusable caching commands so that when a unified CI job is
retried, only the tests that actually failed re-run. Tests that
already passed are skipped via a per-commit cache key.

New commands:
- restore-test-run-cache / save-test-run-cache: per-test cache
- run-test-with-cache: wraps test execution, never fails the step,
  tracks status, saves cache only on success
- report-test-result: surfaces PASSED/SKIPPED/FAILED (when: always)

The cache directory is explicitly removed on failure to prevent
burning the immutable CircleCI cache key with an empty entry.

Also adds cache-aware conditionals to the iOS 14 background simulator
installation (skips download when iOS 14 tests are cached).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ajpallares