Fix MaintenanceModeTest test failures

The MaintenanceModeTest test occasionally fails, e.g.: https://git.drupalcode.org/project/metatag/-/jobs/2265753

Drupal\Tests\metatag\Functional\MaintenanceModeTest 0 passes 1 exceptions
FATAL Drupal\Tests\metatag\Functional\MaintenanceModeTest: test runner returned a non-zero error code (2).
Drupal\Tests\metatag\Functional\MaintenanceModeTest 0 passes 1 fails

---- Drupal\Tests\metatag\Functional\MaintenanceModeTest ----
Status    Group      Filename          Line Function                            
--------------------------------------------------------------------------------
Exception Other      phpunit-6.xml        0 Drupal\Tests\metatag\Functional\Mai
    PHPUnit Test failed to complete; Error: PHPUnit 9.6.20 by Sebastian
    Bergmann and contributors.
    
    Testing Drupal\Tests\metatag\Functional\MaintenanceModeTest
    E                                                                   1 / 1
    (100%)
    
    Time: 00:07.916, Memory: 4.00 MB
    
    There was 1 error:
    
    1) Drupal\Tests\metatag\Functional\MaintenanceModeTest::testUser1
    Behat\Mink\Exception\ExpectationException: The string "Access denied | "
    appears in the HTML response of this page, but it should not.
    
    /builds/project/metatag/vendor/behat/mink/src/WebAssert.php:888
    /builds/project/metatag/vendor/behat/mink/src/WebAssert.php:380
    /builds/project/metatag/web/core/tests/Drupal/Tests/WebAssert.php:566
    /builds/project/metatag/tests/src/Functional/MaintenanceModeTest.php:59
    /builds/project/metatag/vendor/phpunit/phpunit/src/Framework/TestResult.php:729
    
    ERRORS!
    Tests: 1, Assertions: 3, Errors: 1.
    
    HTML output was generated
    http://localhost/web/sites/simpletest/browser_output/Drupal_Tests_metatag_Functional_MaintenanceModeTest-1-83011154.html
    http://localhost/web/sites/simpletest/browser_output/Drupal_Tests_metatag_Functional_MaintenanceModeTest-2-83011154.html
Fail      run-tests. Unknown              0 Unknown                            
    FATAL Drupal\Tests\metatag\Functional\MaintenanceModeTest: test runner
    returned a non-zero error code (2).