add mock source

client/lib/source/mock-source.ts

@@ -0,0 +1,14 @@
+import { Observable } from 'rxjs'
+import { Update } from '../types'
+import { Source } from './source'
+
+export class MockSource<T> implements Source<T> {
+
+  constructor (
+    private readonly seed: Observable<Update<T>>,
+  ) { }
+
+  watch$ (): Observable<Update<T>> {
+    return this.seed
+  }
+}