Add async iteration test for pure version

Author: slowcheetah

tests/type-definitions/pure/proposals/async-iteration.es2018.test.ts

@@ -0,0 +1,12 @@
+import $Symbol from '@core-js/pure/full/symbol/index';
+
+async function* makeAsyncIterable() {
+  yield 1;
+  yield 2;
+}
+
+for await (const x of {
+  [$Symbol.asyncIterator]: makeAsyncIterable,
+}) {
+  const n: number = x;
+}