Storybook: Add basic e2e verification test (#89779)

* add very basic tests to verify storybook builds correctly

* add storybook step to drone

* reorder steps

* drone tweaks

* don't need host since it's set in env

* don't need to wait

* format build.star and readd wait

* install netcat in CI

* do a yarn install here to get correct bindings

* refactoring to hopefully work better in CI

* add wait-on

* add verbose logging

* localhost?

* more logging

* specify storybook host

* ...

* back to grafana-server

* does this work? 🤔

* run storybook e2e test after rgm-package so the backround process is running for less time

* split into separate step

* format

e2e/storybook/verify.spec.ts

@@ -0,0 +1,14 @@
+// very basic test to verify that the button story loads correctly
+// this is only intended to catch some basic build errors with storybook
+// NOTE: storybook must already be running (`yarn storybook`) for this test to work
+describe('Verify storybook', () => {
+  it('Loads the button story correctly', () => {
+    cy.visit('?path=/story/buttons-button--basic');
+    getIframeBody().find('button:contains("Example button")').should('be.visible');
+  });
+});
+
+// see https://www.cypress.io/blog/2020/02/12/working-with-iframes-in-cypress
+function getIframeBody() {
+  return cy.get('#storybook-preview-iframe').its('0.contentDocument.body').should('not.be.empty').then(cy.wrap);
+}