Fix incorrect assert failure reporting.

	Fix incorrect assert failure reporting.

	* grub-core/tests/example_functional_test.c (example_test): Add
	a failure comment.
	* grub-core/tests/lib/test.c (add_failure): Renamed to ...
	(failure_start): ...this. Check that malloc succeeded.
	Don't call xvasprintf. Return failure struct.
	(failure_append_vtext): New function.
	(failure_append_text): Likewise.
	(add_failure): Likewise.
	(grub_test_assert_helper): Likewise.
	* include/grub/test.h (grub_test_assert_helper): New declaration.
	(grub_test_assert): Macro rewritten.