New nightwatch commands for login and logout

To allow testing the authenticated user workflow via JS tests written in nightwatch following new commands have been added:

• drupalCreateRole
• drupalCreateUser
• drupalLogin
• drupalLoginAsAdmin
• drupalLogout
• drupalUserIsLoggedIn

Usage:

These commands can be used like this:

// core/tests/Drupal/Nightwatch/Tests/loginTest.js
module.exports = {
  '@tags': ['core'],

  before(browser) {
    browser
      .drupalInstall();
  },
  after(browser) {
    browser
      .drupalUninstall();
  },

  'Test login': (browser) => {
    browser
      .drupalCreateUser({
        name: 'user',
        password: '123',
        permissions: ['access site reports'],
      })
      .drupalLogin({ name: 'user', password: '123' })
      .drupalRelativeURL('/admin/reports')
      .expect.element('h1.page-title').text.to.contain('Reports');
  },
};