use gcovinator instead of gcovr to generate gcov HTML files

Makefile

@@ -9,5 +9,5 @@ clean:
 test:
 	./waf build --targets tests
 	./build/tests
-	@mkdir -p coverage
-	-@(cd build; gcovr --object-directory=src/core --root=.. -o ../coverage/index.html --html --html-details --gcov-filter='.*src#core.*')
+	@-rm -rf coverage
+	-gcovinator -b build -s src