test(remember): add test for rememember()

Add a simple test that ensures values the last value is played back.

tests/operator/remember.ts

@@ -0,0 +1,21 @@
+import xs from '../../src/index';
+import {emptyObserver} from '../../src/utils/emptyObserver';
+import * as assert from 'assert';
+
+describe.only('Stream.prototype.remember', () => {
+  it('should replay the last event to a new observer', (done) => {
+    const stream = xs.interval(1).take(6).remember();
+
+    stream.subscribe(emptyObserver);
+
+    setTimeout(() => {
+      stream.subscribe({
+        next(value) {
+          assert.strictEqual(value, 5);
+        },
+        error: done.fail,
+        end: () => done(),
+      });
+    }, 15);
+  });
+});