Fix test assert macro calls

The assert() macro in test is not available anymore. It is superseeded
by TEST_HELPER_ASSERT().
diff --git a/tests/suites/target_test.function b/tests/suites/target_test.function
index d430d9d..b097589 100644
--- a/tests/suites/target_test.function
+++ b/tests/suites/target_test.function
@@ -75,7 +75,7 @@
     c[1] = greentea_getc();
     c[2] = '\0';
 
-    assert( unhexify( &byte, c ) != 2 );
+    TEST_HELPER_ASSERT( unhexify( &byte, c ) != 2 );
     return( byte );
 }
 
@@ -100,7 +100,7 @@
                            greentea_getc(),
                            '\0'
                          };
-    assert( unhexify( &value, c ) != 8 );
+    TEST_HELPER_ASSERT( unhexify( &value, c ) != 8 );
     return( (uint32_t)value );
 }