Test coverage for \Drupal\Core\Updater classes

Problem/Motivation

Followup to #3150990: Updater::install() crashes on file transfer exceptions., which fixed a syntax bug for \Drupal\Core\Updater::install().

This class should have more (unit) test coverage. Specifically we need to ensure that if we made the same mistake a test would fail.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet