check runtime errors (#9791)

Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: Rich-Harris

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

@@ -305,6 +305,11 @@ async function run_test_variant(
 						compileOptions
 					});
 				}
+
+				if (config.runtime_error && !unhandled_rejection) {
+					unintended_error = true;
+					assert.fail('Expected a runtime error');
+				}
 			} finally {
 				instance.$destroy();
 				assert_html_equal(