Add LMS import/export negative 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 860304e..709db8c 100644
--- a/tests/suites/test_suite_lms.function
+++ b/tests/suites/test_suite_lms.function
@@ -157,7 +157,8 @@
     unsigned char *exported_pub_key = NULL;
 
     mbedtls_lms_public_init(&ctx);
-    TEST_EQUAL( mbedtls_lms_import_public_key( &ctx, pub_key->x, pub_key->len ), 0 );
+    TEST_EQUAL( mbedtls_lms_import_public_key( &ctx, pub_key->x, pub_key->len ),
+                expected_import_rc );
 
     if( expected_import_rc == 0 )
     {