Add test for yield/await in class decorators

NOTE: This doesn't work for decorators inside the class body, like in
      babel#8300

packages/babel-plugin-proposal-decorators/test/fixtures/transformation/class-decorators-yield-await/input.js

@@ -0,0 +1,5 @@
+async function* f() {
+  @(yield dec1)
+  @(await dec2)
+  class A {}
+}

packages/babel-plugin-proposal-decorators/test/fixtures/transformation/class-decorators-yield-await/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-decorators", "proposal-class-properties", "external-helpers", "syntax-async-generators"]
+}

packages/babel-plugin-proposal-decorators/test/fixtures/transformation/class-decorators-yield-await/output.js

@@ -0,0 +1,17 @@
+async function* f() {
+  let A = babelHelpers.decorate([yield dec1, await dec2], function (_initialize) {
+    "use strict";
+
+    class A {
+      constructor() {
+        _initialize(this);
+      }
+
+    }
+
+    return {
+      F: A,
+      d: []
+    };
+  });
+}