[embeddable] add should_fetch unit test (#161596)

While messing around with time slider control and custom time ranges, I
noticed shouldFetch$ is triggered (turns out search session id is
changing - maybe a separate issue).

During investigation, I found it difficult to explore without unit test
for shouldFetch$.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

src/plugins/embeddable/public/lib/filterable_embeddable/should_fetch.test.tsx

@@ -0,0 +1,65 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { BehaviorSubject, Subscription } from 'rxjs';
+import type { FilterableEmbeddableInput } from './types';
+import { shouldFetch$ } from './should_fetch';
+
+describe('shouldFetch$', () => {
+  let shouldFetchCount = 0;
+  let subscription: Subscription;
+  let updateInput: (inputFragment: Partial<FilterableEmbeddableInput>) => void;
+  beforeAll(() => {
+    let input: FilterableEmbeddableInput = {
+      id: '1',
+      timeRange: {
+        to: 'now',
+        from: 'now-15m',
+      },
+    };
+    const subject = new BehaviorSubject(input);
+    updateInput = (inputFragment: Partial<FilterableEmbeddableInput>) => {
+      input = {
+        ...input,
+        ...inputFragment,
+      };
+      subject.next(input);
+    };
+
+    subscription = shouldFetch$<FilterableEmbeddableInput>(subject, () => {
+      return input;
+    }).subscribe(() => {
+      shouldFetchCount++;
+    });
+  });
+
+  afterAll(() => {
+    subscription.unsubscribe();
+  });
+
+  test('should not fire on initial subscription', () => {
+    expect(shouldFetchCount).toBe(0);
+  });
+
+  test('should not fire when there are no changes', () => {
+    const initialCount = shouldFetchCount;
+    updateInput({});
+    expect(shouldFetchCount).toBe(initialCount);
+  });
+
+  test('should fire when timeRange changes', () => {
+    const initialCount = shouldFetchCount;
+    updateInput({
+      timeRange: {
+        to: 'now',
+        from: 'now-25m',
+      },
+    });
+    expect(shouldFetchCount).toBe(initialCount + 1);
+  });
+});