Add parallel test execution with pytest-xdist

The test suite was taking 30 minutes to run sequentially. With 434 tests,
parallel execution should significantly reduce CI time.

Changes:
- Add pytest-xdist to requirements-test.txt
- Update Makefile test target to use 'pytest -n auto'
- Auto mode automatically uses number of CPU cores available

Expected improvements:
- Unit tests and model tests can run fully in parallel
- Integration tests with isolated databases can run in parallel
- Overall test time should reduce to ~5-10 minutes on multi-core CI

Note: Crypto RPC tests may still be a bottleneck if they hit shared
wallet daemons, but most other tests should parallelize well.

requirements-test.txt

@@ -1,6 +1,7 @@
 # testing.
 pytest
 pytest-cov
+pytest-xdist  # Parallel test execution
 nose
 mock
 WebTest