Make negative function_id value impossible in test suites

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index 1f4180d..bfc5279 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -174,7 +174,7 @@
  *               DISPATCH_TEST_FN_NOT_FOUND if not found
  *               DISPATCH_UNSUPPORTED_SUITE if not compile time enabled.
  */
-int dispatch_test( int func_idx, void ** params )
+int dispatch_test( size_t func_idx, void ** params )
 {
     int ret = DISPATCH_TEST_SUCCESS;
     TestWrapper_t fp = NULL;
@@ -205,7 +205,7 @@
  *               DISPATCH_TEST_FN_NOT_FOUND if not found
  *               DISPATCH_UNSUPPORTED_SUITE if not compile time enabled.
  */
-int check_test( int func_idx )
+int check_test( size_t func_idx )
 {
     int ret = DISPATCH_TEST_SUCCESS;
     TestWrapper_t fp = NULL;