Fix building of multiple test directories

Makefile

@@ -419,7 +419,7 @@ define BUILD_TEST
     MAKE_TARGET := $2
     COMMAND := $1
     MAKE_CMD := $$(MAKE) -r -R -C $(ROOT_DIR) -f build_test.mk $$(MAKE_TARGET)
-    MAKE_VARS := TEST=$$(TEST_NAME) FULL_TESTS=$$(FULL_TESTS)
+    MAKE_VARS := TEST=$$(TEST_NAME) FULL_TESTS="$$(FULL_TESTS)"
     MAKE_MSG := $$(MSG_MAKE_TEST)
     $$(eval $$(call BUILD))
     ifneq ($$(MAKE_TARGET),clean)