A few config updates to get runtime tests working in CI

[zoecarver]

1. .github/workflows/on-pr.yml - Added hardware tests to PR workflow:

• Added test-hardware job that depends on build
• Added test-hardware to check-all-green needs

2. .github/workflows/call-test-hardware.yml - Fixed test failure handling:

• Removed || true so lit test failures will actually fail CI

3. .github/scripts/run_lit_tests.sh - Set up SYSTEM_DESC_PATH:

• Added logic to find and export SYSTEM_DESC_PATH from ttrt-artifacts/system_desc.ttsys

4. test/lit.cfg.py - Enable hardware tests:

• Removed test_*.py exclusion pattern
• Added tt-mlir's python_packages to PYTHONPATH for ttnn access
• Added ttnn availability detection (REQUIRES: ttnn feature)

5. Test files - Updated directives: test_elementwise_ops.py: UNSUPPORTED: system-darwin → REQUIRES: ttnn and similar for other tests

[zoecarver]

Waiting for #169