test: Add a test for print_buffer()

Add a test for this function, to cover the various features. Expand the
expect_str length to take acount of the ~300-bytes lines generated in one
case.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/test/test.h b/include/test/test.h
index bf7d785..0104e18 100644
--- a/include/test/test.h
+++ b/include/test/test.h
@@ -32,8 +32,8 @@
 	struct udevice *testdev;
 	int force_fail_alloc;
 	int skip_post_probe;
-	char expect_str[256];
-	char actual_str[256];
+	char expect_str[512];
+	char actual_str[512];
 };
 
 /* Test flags for each test */