Add output length test for LMS export too-big test

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 1e5ac42..baf74ab 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 );
+        TEST_EQUAL( exported_pub_key_size,
+                    MBEDTLS_LMS_PUBLIC_KEY_LEN(MBEDTLS_LMS_SHA256_M32_H10 ) );
         ASSERT_COMPARE( pub_key->x, pub_key->len,
                         exported_pub_key, exported_pub_key_size );
         mbedtls_free(exported_pub_key);