chore: make test use multiple cores. (#2696)

In the move to PDM in #2357 `make test` was modified to run `pytest tests` from `pytest tests -n auto`.

This appears to have been inadvertent, so this PR changes it back.

Makefile

@@ -115,7 +115,7 @@ coverage:  											## Run the tests and generate coverage report
 .PHONY: test
 test:  												## Run the tests
 	@echo "=> Running test cases"
-	@$(ENV_PREFIX)pytest tests
+	@$(ENV_PREFIX)pytest tests -n auto
 	@echo "=> Tests complete"
 
 .PHONY: test-examples