[cypress/commands.js] Add "wait" for menuItems

By using a `cy.get` with this command, it forces a `wait` for the DOM to
load with the menu.  Using `cy.window` doesn't (seem to) do that.

This probably could be smaller, but this is a workaround to fix the
currently breaking tests using this method (currently only
`menu.spec.js`).

cypress/support/commands.js

@@ -70,6 +70,7 @@ Cypress.Commands.add("menu", (...items) => {
 
 // cy.menuItems() - returns an array of top level menu items with {title, href, items (array of children)}
 Cypress.Commands.add("menuItems", () => {
+  cy.get('#main-menu nav.primary'); // Wait for menu to appear
   return cy.window().then((window) => window.ManageIQ.menu);
 });