Random fails in TimestampAgoFormatterTest and DateTimeTimeAgoFormatterTest

Problem/Motivation

https://www.drupal.org/pift-ci-job/1210029 #3032376: Files renamed by \_file_save_upload_single() do not have the correct filename on the File entity
https://www.drupal.org/pift-ci-job/1210030 #3032620: \Drupal\file\Plugin\rest\resource\FileUploadResource uses basename() when it needs to use the Drupal version
...

Random fails in:
Drupal\Tests\datetime\Functional\TimestampAgoFormatterTest::testSettings()
Drupal\Tests\datetime\Functional\DateTimeTimeAgoFormatterTest::testSettings()

1) Drupal\FunctionalTests\Datetime\TimestampAgoFormatterTest::testSettings
Behat\Mink\Exception\ResponseTextException: The text "ends in 1 year 1 month 1 week" was not found anywhere in the text of the current page.

/var/www/html/vendor/behat/mink/src/WebAssert.php:787
/var/www/html/vendor/behat/mink/src/WebAssert.php:262
/var/www/html/core/tests/Drupal/FunctionalTests/Datetime/TimestampAgoFormatterTest.php:118
...
1) Drupal\Tests\datetime\Functional\DateTimeTimeAgoFormatterTest::testSettings
Behat\Mink\Exception\ResponseTextException: The text "ends in 1 year 1 month 1 week" was not found anywhere in the text of the current page.

/var/www/html/vendor/behat/mink/src/WebAssert.php:787
/var/www/html/vendor/behat/mink/src/WebAssert.php:262
/var/www/html/core/modules/datetime/tests/src/Functional/DateTimeTimeAgoFormatterTest.php:118

Proposed resolution

Fix the tests to not randomly fail.

Remaining tasks

1. Debug the randomness
2. Fix it
3. Review
4. RTBC
5. Commit

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.