Fix strict standards for the children of ViewUnitTestBase::setUpFixtures()

Problem/Motivation

A few tests are failing on the temporary SQLite testbot with this error:

Strict Standards: Declaration of Drupal\views\Tests\Plugin\RelationshipJoinTestBase::setUpFixtures() should be compatible with Drupal\views\Tests\ViewUnitTestBase::setUpFixtures($import_test_views = true) in /var/www/core/modules/views/src/Tests/Plugin/RelationshipJoinTestBase.php on line 18

Proposed resolution

Fix the test classes.

Remaining tasks

None.

User interface changes

Nope.

API changes

Nope.