test: Add testAsync() helper to Suite

Add testAsync() helper function to Suite to simplify running tests that
return a Promise. The test action is executed and if a syncronous error
is thrown then it is immediately considered failed. If the Promise resolves
successfully then the test is considered successful. If the Promise
rejects with an Error then the test is considered failed.

test/suite.js

@@ -72,6 +72,19 @@ class Suite {
     const test = new Test(name, cb)
     this._queue.push(test)
   }
+
+  testAsync (name, action) {
+    const test = new Test(name, cb => {
+      try {
+        Promise.resolve(action())
+          .then(() => cb(null))
+          .catch((err) => cb(err))
+      } catch (err) {
+        cb(err)
+      }
+    })
+    this._queue.push(test)
+  }
 }
 
 process.on('unhandledRejection', (e) => {