Make it easy to test just a subset of tests

with e.g., 'make TEST_EXTRA="extmod/uasyncio*.py" test'

Author: jepler

ports/unix/Makefile

@@ -296,9 +296,10 @@ include $(TOP)/py/mkrules.mk
 
 .PHONY: test test_full
 
+TEST_EXTRA ?=
 test: $(PROG) $(TOP)/tests/run-tests.py
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))
-	cd $(TOP)/tests && MICROPY_MICROPYTHON=../$(DIRNAME)/$(PROG) ./run-tests.py
+	cd $(TOP)/tests && MICROPY_MICROPYTHON=../$(DIRNAME)/$(PROG) ./run-tests.py $(TEST_EXTRA)
 
 test_full: $(PROG) $(TOP)/tests/run-tests.py
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))