use eager_stackful_preorder_iter in variable_set, add term_variables/1 tests

tests/scryer/src_tests.rs

@@ -93,3 +93,12 @@ fn ground_tests() {
         "All tests passed",
     );
 }
+
+#[test]
+fn term_variables_tests() {
+    run_top_level_test_with_args(
+        &["src/tests/term_variables.pl", "-f", "-g", "main_quiet"],
+        "",
+        "All tests passed",
+    );
+}