test: create a async/await friendly rollup watch wrapper

Because I'm tired of callbacks.

package.json

@@ -42,6 +42,7 @@
     "jest": "^24.8.0",
     "jest-cli": "^24.8.0",
     "lerna": "^3.16.4",
+    "p-defer": "^3.0.0",
     "pegjs": "0.10.0",
     "read-dir-deep": "^6.0.0",
     "rollup": "^1.11.3",

packages/test-utils/rollup-watching.js

@@ -1,5 +1,7 @@
 "use strict";
 
+const defer = require("p-defer");
+
 module.exports = (cb) => {
     let count = 0;
 
@@ -17,3 +19,32 @@ module.exports = (cb) => {
         cb(count, details);
     };
 };
+
+module.exports.promise = (watcher) => {
+    let deferred;
+
+    watcher.on("event", (e) => {
+        console.log("watch event", e.code);
+
+        if(e.code === "ERROR" || e.code === "FATAL") {
+            return deferred && deferred.reject(e.error);
+        }
+
+        if(e.code !== "END") {
+            // eslint-disable-next-line consistent-return
+            return;
+        }
+
+        return deferred && deferred.resolve(e);
+    });
+
+    const out = () => {
+        deferred = defer();
+
+        return deferred.promise;
+    };
+
+    out.close = () => watcher.close();
+
+    return out;
+};