Enforce strict types in tests

Current status (assuming Kernel tests gets in soon):

<include-pattern>*/tests/*</include-pattern>
<exclude-pattern>*/tests/modules/*</exclude-pattern>
<exclude-pattern>*/tests/themes/*</exclude-pattern>
<exclude-pattern>*/tests/*_test/*</exclude-pattern>
<exclude-pattern>*/tests/*fixture*/*</exclude-pattern>

• 📌 Add declare(strict_types=1) to all test modules Postponed should take care of the first two exclude patterns.
• We need another issue for test themes, I think they are all in core/modules/system/tests/themes/
• We need another issue for fixtures, although maybe we should continue to ignore them

That leaves the following that need to be categorised:

core/modules/comment/src/Tests/CommentTestTrait.php
core/modules/contact/tests/drupal-7.contact.database.php
core/modules/filter/tests/filter_test_plugin/src/Plugin/Filter/FilterSparkles.php
core/modules/filter/tests/filter_test_plugin/src/Plugin/Filter/FilterTestStatic.php
core/modules/menu_link_content/tests/menu_link_content_dynamic_route/src/Routes.php
core/modules/migrate_drupal/src/Tests/StubTestTrait.php
core/modules/node/tests/node_access_test_auto_bubbling/src/Controller/NodeAccessTestAutoBubblingController.php
core/modules/system/tests/http.php
core/modules/system/tests/https.php
core/modules/views/src/Tests/AssertViewsCacheTagsTrait.php
core/modules/views/src/Tests/TestHelperPlugin.php
core/modules/views/src/Tests/ViewResultAssertionTrait.php
core/modules/views/src/Tests/ViewTestData.php
core/tests/bootstrap.php

• Possibly the Traits and Views classes should move to the tests dir in each module, unless there is some reason they need to live in src/Tests instead.
• We should make sure to capture filter_test_plugin, menu_link_content_dynamic_route and node_access_test_auto_bubbling in 📌 Add declare(strict_types=1) to all test modules Postponed , or move them to a test/modules directory first
• Not too sure about the remaining non-class php files, I guess we should be ok to include them