split out task to build unit tests to "build_tests" task

Makefile

@@ -9,9 +9,12 @@ ifneq ($(FILTER),)
   GTEST_FILTER := --gtest_filter=$(FILTER)
 endif
 
-.PHONY: test
-test:
+.PHONY: build_tests
+build_tests:
 	./waf build --targets tests
+
+.PHONY: test
+test: build_tests
 	./build/tests $(GTEST_FILTER)
 	@-rm -rf coverage
 	-gcovinator -b build -s src