tests/build_system_utils: add test for build system utilities

This will allow testing the build system 'utils' functions.

tests/build_system_utils/Makefile

+BOARD_WHITELIST = native
+
+include ../Makefile.tests_common
+include $(RIOTBASE)/Makefile.include
+
+
+# Test utils commands
+define command_should_fail
+$1 2>/dev/null && { echo "Command '$1' should have failed but did not" >&2; $1; exit 1; } || true
+endef
+
+define command_should_succeed
+$1 || { echo "Command '$1' failed" >&2; $1; exit 1; }
+endef
+
+
+MAKEFILES_UTILS = $(RIOTMAKE)/utils
+
+TESTS = test-ensure_value test-ensure_value-negative
+
+# Tests will be run both in the host machine and in `docker`
+all: build-system-utils-tests
+
+build-system-utils-tests: $(TESTS)
+.PHONY: build-system-utils-tests $(TESTS)
+
+
+# tests for 'ensure_value'
+test-ensure_value:
+	$(Q)$(call command_should_succeed,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-checks.mk test-ensure_value)
+
+test-ensure_value-negative:
+	$(Q)$(call command_should_fail,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-checks.mk test-ensure_value-negative)

tests/build_system_utils/README.md

+Build system utils tests
+========================
+
+This test checks the build system 'utils' functions declared in
+`makefiles/utils`.
+
+The test output says nothing in case of success.
+
+Note
+----
+
+It should not be necessary to compile but this simplifies the integration in
+murdock for the moment. Also there will be other tests that may need to check
+the result of the compilation.

tests/build_system_utils/main.c

+/*
+ * Copyright (C) 2018 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser General
+ * Public License v2.1. See the file LICENSE in the top level directory for more
+ * details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Empty main file
+ *
+ * @author      Gaëtan Harter <gaetan.harter@fu-berlin.de>
+ *
+ * @}
+ */
+
+int main(void)
+{
+    /* The important rules are in the Makefile */
+    return 0;
+}