efi_selftest: test FatToStr() truncation

Let the FatToStr test check that the FatSize parameter is considered.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/lib/efi_selftest/efi_selftest_unicode_collation.c b/lib/efi_selftest/efi_selftest_unicode_collation.c
index 11050a3..32c99ca 100644
--- a/lib/efi_selftest/efi_selftest_unicode_collation.c
+++ b/lib/efi_selftest/efi_selftest_unicode_collation.c
@@ -178,7 +178,7 @@
 
 	boottime->set_mem(str, sizeof(str), 0);
 	unicode_collation_protocol->fat_to_str(unicode_collation_protocol, 6,
-					       "U-BOOT", str);
+					       "U-BOOT!", str);
 	if (efi_st_strcmp_16_8(str, "U-BOOT")) {
 		efi_st_error("fat_to_str returned \"%ps\"\n", str);
 		return EFI_ST_FAILURE;