Add minimal unit tests for Concast class.

The Concast class is heavily exercised by much of our test suite, so I
would not say it is currently untested, but I would love to expand on
these Concast-specific unit tests in the future.

src/utilities/observables/__tests__/Concast.ts

@@ -0,0 +1,32 @@
+import { itAsync } from "../../../testing/core";
+import { Observable } from "../Observable";
+import { Concast } from "../Concast";
+
+describe("Concast Observable (similar to Behavior Subject in RxJS)", () => {
+  itAsync("can concatenate other observables", (resolve, reject) => {
+    const concast = new Concast([
+      Observable.of(1, 2, 3),
+      Promise.resolve(Observable.of(4, 5)),
+      Observable.of(6, 7, 8, 9),
+      Promise.resolve().then(() => Observable.of(10)),
+      Observable.of(11),
+    ]);
+
+    const results: number[] = [];
+    concast.subscribe({
+      next(num) {
+        results.push(num);
+      },
+
+      error: reject,
+
+      complete() {
+        concast.promise.then(finalResult => {
+          expect(results).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]);
+          expect(finalResult).toBe(11);
+          resolve();
+        }).catch(reject);
+      },
+    });
+  });
+});