test: Support testing malloc() failures

It is helpful to test that out-of-memory checks work correctly in code
that calls malloc().

Add a simple way to force failure after a given number of malloc() calls.

Fix a header guard to avoid a build error on sandbox_vpl.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/test/test-main.c b/test/test-main.c
index 90a324b..7a38716 100644
--- a/test/test-main.c
+++ b/test/test-main.c
@@ -47,6 +47,7 @@
 	uts->force_fail_alloc = false;
 	uts->skip_post_probe = false;
 	gd->dm_root = NULL;
+	malloc_disable_testing();
 	if (CONFIG_IS_ENABLED(UT_DM) && !CONFIG_IS_ENABLED(OF_PLATDATA))
 		memset(dm_testdrv_op_count, '\0', sizeof(dm_testdrv_op_count));
 	arch_reset_for_test();