itest: runner must call on_finished deferred

follow godot-rust/gdext#1095

Author: ColinWttt

itest/rust/src/framework/runner.rs

@@ -109,7 +109,7 @@ impl IntegrationTests {
                 );
 
                 // Calling deferred to break a potentially synchronous call stack and avoid re-entrancy.
-                on_finished.call(&[result.to_variant()]);
+                on_finished.call_deferred(&[result.to_variant()]);
             };
 
             Self::run_async_rust_tests(