Clarify case when high level error code is incorrect

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 72b886d..b7c9867 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -309,7 +309,7 @@
     }
     else if ( high < -0x7F80 )
     /* high > 0111111110000000
-     * Error code is larger than the greatest high + module level error.
+     * Error code is greater than the largest allowed high level module ID.
      */
     {
         mbedtls_test_fail( "'high' error code is greater than 15 bits",