Fail build when test fail (sonic-net#53)

Why I did it
In current PR check, we would build dhcp6relay debian package, but even if the unit test failed, the build would still be ongoing, then the PR check would pass. It would cause we miss some unexpected damage change
image

How I did it
Modify Makefile to exit with code 1 if test fail

How I verify it
PR check

Makefile

@@ -49,7 +49,7 @@ $(DHCP6RELAY_TEST_TARGET): $(TEST_OBJS)
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) $(LDLIBS_TEST) -o $@
 
 test: $(DHCP6RELAY_TEST_TARGET)
-	sudo ASAN_OPTIONS=detect_leaks=0 ./$(DHCP6RELAY_TEST_TARGET) --gtest_output=xml:$(DHCP6RELAY_TEST_TARGET)-test-result.xml || true
+	sudo ASAN_OPTIONS=detect_leaks=0 ./$(DHCP6RELAY_TEST_TARGET) --gtest_output=xml:$(DHCP6RELAY_TEST_TARGET)-test-result.xml || { echo 'Tests failed'; exit 1; }
 	$(GCOVR) -r ./ --html --html-details -o $(DHCP6RELAY_TEST_TARGET)-code-coverage.html
 	$(GCOVR) -r ./ --xml-pretty -o $(DHCP6RELAY_TEST_TARGET)-code-coverage.xml