Prevent uncaught errors to make test fails when they are expected and are fired during template effects flush

packages/svelte/tests/runtime-legacy/shared.ts

@@ -450,13 +450,14 @@ async function run_test_variant(
 					'Expected component to unmount and leave nothing behind after it was destroyed'
 				);
 
-				// TODO: This seems useless, unhandledRejection is only triggered on the next task
-				// by which time the test has already finished and the next test resets it to null above
+				// uncaught errors like during template effects flush
 				if (unhandled_rejection) {
+					if (!config.expect_unhandled_rejections) {
 						throw unhandled_rejection; // eslint-disable-line no-unsafe-finally
 					}
 				}
 			}
+		}
 	} catch (err) {
 		if (config.runtime_error) {
 			assert.include((err as Error).message, config.runtime_error);