Makefile: Add test checking.

Signed-off-by: zhanglinjing <Linjing.Zhang@infineon.com>

Author: LinjingZhang

Makefile

@@ -120,6 +120,9 @@ test_onewire_DS18x20: TESTS=-DTEST_ONEWIRE_DS18x20
 compile:
 ifeq ($(FQBN),)
 	$(error "Must set variable FQBN in order to be able to compile Arduino sketches !")
+endif
+ifeq ($(TESTS),)
+    $(error "Must set variable TESTS in order to be able to flash a specific arduino core test !")
 else
 	arduino-cli compile --clean --log --warnings all --fqbn $(FQBN) \
 		                    --build-property compiler.c.extra_flags="\"-DUNITY_INCLUDE_CONFIG_H=1\"" \