Add declare(strict_types=1) to all Build tests

Problem/Motivation

This is a child issue of #3376057: [META] Add declare(strict_types = 1) to all tests.

Steps to reproduce

Add to phpcs.xml.dist:

<rule ref="SlevomatCodingStandard.TypeHints.DeclareStrictTypes">
    <properties>
      <property name="spacesCountAroundEqualsSign" value="0" />
    </properties>
    <include-pattern>./tests/Drupal/BuildTests/*</include-pattern>
  </rule>

Run phpcs:

composer phpcs

Proposed resolution

Run phpcbf:

composer phpcbf

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet