#10 Add parallel multi-case test suite runner

[iray-tno]

Summary

• Add prepkit test suite <file> <cases_dir> for multi-case exact-match testing.
• Compile the solver once, then run matching *.in/*.out cases in parallel with --workers.
• Report per-case PASS/FAIL, runtime, and aggregate summary.
• Preserve existing prepkit test <file> behavior through the command dispatcher.

Tests

• uv run pytest tests/test_test_command.py tests/test_cli.py::TestTestCommand -q
• uv run pytest -q

Closes #10.