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/targets/corstone-300/CMakeLists.txt b/targets/corstone-300/CMakeLists.txt
index 3a6f291..a409a97 100644
--- a/targets/corstone-300/CMakeLists.txt
+++ b/targets/corstone-300/CMakeLists.txt
@@ -64,6 +64,7 @@
         -t corstone-300
         -a ethos-${ETHOSU_ARCH}
         -m ${ETHOSU_NUM_MACS}
+        $<$<BOOL:ETHOSU_CTEST_TARMAC_TRACE>:--tarmac>
     CACHE INTERNAL "Default test command")
 
 # Enable trustzone support in core_software