Makefile: Remove redundancy.

Author: LinjingZhang

Makefile

@@ -124,9 +124,6 @@ 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\"" \