feat: add commands to assert success messages, error messages and page titles

[kolaente]

This PR adds a few short helper functions:

• shouldHavePageTitle
• shouldHaveSuccessMessage
• shouldHaveErrorMessage

All of them pretty much do what their title says.

Example usage for checking if a success message is present:

import {shouldHaveSuccessMessage} from '../support/utils'

it('Can add a product to the cart from the product page', () => {
    cy.get(selectors.addToCartButton).click()
    shouldHaveSuccessMessage(`You added ${product.simpleProductName} to your shopping cart.`)
})

[kolaente]

Not sure how much these selectors differ between hyvä and luma? I've used the commands in luma only so far, so I can't say a lot about hyvä.

[peterjaap]

Could you move the global commands to helper functions? See the first bullet point under Contributing; https://github.com/elgentos/magento2-cypress-testing-suite#contributing

Global commands are bad mmkay.

[kolaente]

Should I remove the login command as well while I'm at it? Looks like it's not used anywhere.

[peterjaap]

Yes please!

[kolaente]

Done!

[peterjaap]

Awesome, thanks!