The old code gives arguments to a printf function which can't work

	The old code gives arguments to a printf function which can't work
	correctly, and the compiler complains.

	* grub-core/tests/example_functional_test.c (example_test): Add
	missing text.
	* grub-core/tests/lib/test.c (add_failure): Rewrite.
	* include/grub/test.h (grub_test_assert_helper): New declaration.
	(grub_test_assert): Use grub_test_assert_helper.