docs: Add mock_template documentation to docs.

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docs/testing/testing-with-node.md

@@ -153,6 +153,68 @@ The test runner (`index.js`) automatically runs all .js files in the
 `web/tests` directory, so you can simply start editing a file
 in that directory to create a new test.
 
+## Verifying HTML templates with `mock_template`
+
+As a project, we prefer [end-to-end
+testing][testing-philosophy-end-to-end] where possible, since those
+tests are usually both more effective at catching bugs and cheaper to
+maintain than tests that make heavy use of mocks.
+
+One place where mocks can often be useful is when testing logic for
+rendering an HTML template in Zulip. The interesting logic that one
+wants to verify can be split between two places:
+
+- Computing the **context data** object passed into the HTML template.
+- Conditional logic in the HTML template itself.
+
+It can work well to write tests that verify properties of the computed
+HTML template, for example, is a given CSS class present in the
+result.
+
+But often, one can write a more readable test by instead verifying
+the values of parameters in the context passed into the template
+rendering. The `mock_template` function in Zulip's testing library
+is designed to support this this.
+
+We use `mock_template` in our unit tests to verify that the JS code is
+calling the template with the expected context data. And then we use
+the results of mock_template to supply the JS code with either the
+actual HTML from the template or some kind of zjquery stub.
+
+The `mock_template` function accepts 3 parameters:
+
+- The path within `web/templates` to the [Handlebars
+  template](../subsystems/html-css.md) that you'd like to mock.
+- Whether to call the actual template rendering function so that you
+  can verify the HTML generated by this specific template. Since
+  Handlebars rendering in tests takes time and rarely catches bugs, we
+  recommend using `false` if you're only planning to check the context
+  data.
+- A callback function that you can use to include assertions about
+  what parameters were passed into the template. This function
+  receives a `data` parameter, with the context data, and an `html`
+  parameter if the real template was rendered.
+
+The following illustrates the two common patterns for using this method.
+
+```js
+run_test("test something calling template", ({mock_template}) => {
+    mock_template("path/to/template.hbs", false, (data) => {
+        assert.deepEqual(data, {...};
+        // or assert.deepEqual(data.foo, {...});
+        return "stub-for-zjquery";
+    });
+
+    mock_template("path/to/template.hbs", true, (data, html) => {
+        assert.deepEqual(data, {...};
+        assert.ok(html.startWith(...));
+        return html;
+    });
+});
+```
+
+[testing-philosophy-end-to-end]: https://zulip.readthedocs.io/en/stable/testing/philosophy.html#integration-testing-or-unit-testing
+
 ## Coverage reports
 
 You can automatically generate coverage reports for the JavaScript unit