resident: Fixed syntax error with g++ compilation

A function used in a test was not returning a value, while an int was
expected so the test would fail to compile.

tests/resident.at: Return the result of atexit().

tests/resident.at

@@ -116,7 +116,7 @@ extern "C"
 int
 setup_plugin (void)
 {
-  atexit (bye);
+  return atexit (bye);
 }
 ]])