Docs: Add comprehensive test suite documentation (#106)

Catalogs all 318 automated tests organized by module:
- Admin panel (70 tests)
- Authentication (21 tests)
- Jobs API (28 tests)
- Saved jobs (17 tests)
- Models (49 tests)
- Stats (8 tests)
- Robots.txt (26 tests)
- Scraper utilities (44 tests)
- UltiPro/UKG scraper (34 tests)
- ADP scraper (7 tests)
- AI analyzer (7 tests)
- Playwright fallback (7 tests)

Includes test descriptions and instructions for running tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: mbuckingham74