Add test for controlling height between states

The button shouldn't change its vertical position
when the state changes. The text confirming the
copy is just one line so setting height for both
based on the API key, which can run to 2 lines
makes sense.

Explained in this PR:

https://github.com/alphagov/notifications-admin/pull/2428

tests/javascripts/apiKey.test.js

@@ -54,6 +54,8 @@ describe('API key', () => {
 
   describe("If copy command is available", () => {
 
+    let componentHeightOnLoad;
+
     beforeAll(() => {
 
       // assume copy command is available
@@ -79,6 +81,10 @@ describe('API key', () => {
 
       component = document.querySelector('[data-module=api-key]');
 
+      // mock DOM API called for element height
+      componentHeightOnLoad = 50;
+      jest.spyOn(component, 'offsetHeight', 'get').mockImplementation(() => componentHeightOnLoad);
+
       // start the module
       window.GOVUK.modules.start();
 
@@ -111,6 +117,13 @@ describe('API key', () => {
 
       });
 
+      test("It should set the component's minimum height based on its height when the page loads", () => {
+
+        // to prevent the position of the button moving when the state changes
+        expect(window.getComputedStyle(component)['min-height']).toEqual(`${componentHeightOnLoad}px`);
+
+      });
+
     });
 
     describe("If you click the 'Copy API key to clipboard' button", () => {