Replace $GLOBALS['base_root'] with getAbsoluteUrl() in Functional Test

Problem/Motivation

I was working on the issue #3402601 with module Pagerer.
I believe if in PagetTest.php $GLOBALS['base_root'] was not used, we will not have this issue.
There is only 2 files with using this approach, and I believe using \Drupal\Tests\UiHelperTrait::getAbsoluteUrl() is better.

The problem was that because in Gitlab CI "web" subdrectory is used, we run into issue, having tests failing in Gitlab CI approach.
\Drupal\Tests\UiHelperTrait::getAbsoluteUrl() used in many tests in core. And there are only 3 places where $GLOBALS['base_root'] used.

Steps to reproduce

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Proposed resolution

Replace $GLOBALS['base_root'] with \Drupal\Tests\UiHelperTrait::getAbsoluteUrl() in PhpUnit functional tests.

Remaining tasks

Refactor code.

User interface changes

none

API changes

none

Data model changes

none

Release notes snippet

none