Use sniff DrupalPractice.CodeAnalysis.VariableAnalysis on */tests/*

Problem/Motivation

In #3106216: Remove unused variables from core we introduced the sniff DrupalPractice.CodeAnalysis.VariableAnalysis to detect unused variables in Drupal Core.
To narrow the scope the decision was made to exclude all the test-classes.

This issue is created to make that sniff run without errors on all test-classes.

Steps to reproduce

Proposed resolution

- Remove the line <exclude-pattern>*/tests/*</exclude-pattern> from sniff DrupalPractice.CodeAnalysis.VariableAnalysis in core/phpcs.xml.dist
- Fix all and any errors

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet