test: add an async syntax regression test

- cover the bases and ensure async is supported too, not just generators

- use some syntax from a bug report that would cause async-to-promises
  to produce invalid JS and Rollup to subsequently error out

test/e2e/fixtures/build-default/src/index.ts

@@ -3,6 +3,7 @@ import './syntax/optional-chaining';
 
 import './syntax/jsx-import/JSX-import-JSX';
 
+import './syntax/async';
 export { testGenerator } from './syntax/generator';
 
 export { foo } from './foo';

test/e2e/fixtures/build-default/src/syntax/async.ts

@@ -0,0 +1,6 @@
+// regression test for async/await
+// code inspired by https://github.com/formium/tsdx/issues/869
+(async () => {
+  await Promise.resolve();
+  console.log('a side effect to make sure this is output');
+})();