Create Nightwatch equivalents of jQuery UI dialog qunit tests

Problem/Motivation

In preparation for #2158943: Replace jQuery UI dialog with supported library or polyfill, we should have core equivalents of jQuery UI's dialog tests. This way, we can insure the shimmed dialog continues to pass tests.

This is similar to the tests added in #3076171: Provide a new library to replace jQuery UI autocomplete

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet