Add CMake variable to pass arguments to ctest

This is needed in order to for example turn tarmac tracing on for the
FVP running the tests.

Change-Id: I6606134046c2e228d3fe06db41792e7403cd6ba8
diff --git a/cmake/helpers.cmake b/cmake/helpers.cmake
index c8a4987..c12f427 100644
--- a/cmake/helpers.cmake
+++ b/cmake/helpers.cmake
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2020-2021 Arm Limited. All rights reserved.
+# Copyright (c) 2020-2022 Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -111,6 +111,8 @@
 # Test
 #############################################################################
 
+set(ETHOSU_CTEST_TARMAC_TRACE OFF CACHE BOOL "Enable to generate tarmac trace files when running ctest")
+
 function(ethosu_add_test target)
     if(NOT BUILD_TESTING)
         return()