try adding concurrency test

src/test/queue.spec.ts

@@ -127,6 +127,42 @@ describe("Queue", () => {
       });
   });
 
+  it("should handle tasks in concurrency", () => {
+    const handler = sinon
+      .stub()
+      .rejects(TEST_ERROR)
+      .withArgs("1")
+      .onCall(2)
+      .resolves(0)
+      .withArgs("2")
+      .onCall(2)
+      .resolves(0);
+
+    const q = new Queue({
+      backoff: 0,
+      concurrency: 2,
+      handler,
+      retries: 3,
+    });
+
+    q.add("1");
+    q.add("2");
+    q.close();
+
+    return q
+      .wait()
+      .catch((err: Error) => {
+        throw new Error("handler should have passed " + q.stats());
+      })
+      .then(() => {
+        expect(handler.callCount).to.equal(6);
+        expect(q.complete).to.equal(2);
+        expect(q.success).to.equal(2);
+        expect(q.errored).to.equal(0);
+        expect(q.retried).to.equal(4);
+      });
+  });
+
   it("should retry the number of retries for mutiple tasks", () => {
     const handler = sinon
       .stub()
@@ -140,6 +176,7 @@ describe("Queue", () => {
 
     const q = new Queue({
       backoff: 0,
+      concurrency: 1,
       handler,
       retries: 3,
     });