Add output check to export too-big buffer tests

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/tests/suites/test_suite_lms.function b/tests/suites/test_suite_lms.function
index ff117ea..1e5ac42 100644
--- a/tests/suites/test_suite_lms.function
+++ b/tests/suites/test_suite_lms.function
@@ -186,6 +186,8 @@
         TEST_EQUAL( mbedtls_lms_export_public_key( &ctx, exported_pub_key,
                                                    exported_pub_key_buf_size, NULL ),
                     0 );
+        ASSERT_COMPARE( pub_key->x, pub_key->len,
+                        exported_pub_key, exported_pub_key_size );
         mbedtls_free(exported_pub_key);
         exported_pub_key = NULL;
     }