Build: Change TFM_TEST_REPO_PATH to CONFIG_TFM_TEST_DIR

The TFM_TEST_REPO_PATH is not really "test repo path".
And it's always used as "${TFM_TEST_REPO_PATH}/test".

This patch changes ${TFM_TEST_REPO_PATH}/test to CONFIG_TFM_TEST_DIR.

Change-Id: I60c9e8d2bb4cca4a81198044bf9d47b58532d310
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index d905e30..204f62f 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -34,6 +34,7 @@
 
     if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 8.0.0)
         add_compile_options(
+            # TO-DO: is this required anymore after split build?
             -fmacro-prefix-map=${TFM_TEST_REPO_PATH}=TFM_TEST_REPO_PATH
         )
     endif()