Add tests of $base_url in UiHelperTrait

Problem/Motivation

Add additional, explicit, test coverage to UiHelperTrait to ensure that it works with different values for $base_url.

from catch in #2986962-31: BrowserTestBase::drupalGet() does not appear to be handling base url properly →

There is already existing implicit test coverage, therefor I am setting this it minor.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet