test(e2e): add a simple test for viewport addon

CLI now generates project with `addon-essential` so it enable `addon-viewport` by default.
I added a simple test to check that viewport is properly loaded:
 - Go to Welcome Story
 - Click on Viewport button in the toolbar
 - Select "Small Mobile"
 - Check that Welcome Story is still displayed

cypress/generated/addon-viewport.spec.ts

@@ -0,0 +1,16 @@
+describe('addon-viewport', () => {
+  before(() => {
+    cy.visitStorybook();
+  });
+
+  it('should have viewport button in the toolbar', () => {
+    cy.navigateToStory('button', 'Text');
+
+    // Click on viewport button and select small mobile
+    cy.get('[title="Change the size of the preview"]').click();
+    cy.get('#mobile1').click();
+
+    // Check that Welcome story is still displayed
+    cy.getStoryElement().should('contain.text', 'Button');
+  });
+});