Random fail in EntityReferenceAdminTest

Problem/Motivation

Drupal\Tests\field\Functional\EntityReference\EntityReferenceAdminTest has failed twice in 8.6.x HEAD in the past two weeks:
https://www.drupal.org/pift-ci-job/1179392
https://www.drupal.org/pift-ci-job/1184261

The fail is kind of odd:

Drupal\Tests\field\Functional\EntityReference\EntityReferenceAdminTest
fail: [Other] Line 0 of sites/default/files/simpletest/phpunit-264.xml:
PHPunit Test failed to complete; Error: PHPUnit 6.5.13 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\field\Functional\EntityReference\EntityReferenceAdminTest
...                                                                 3 / 3 (100%)

Time: 54.47 seconds, Memory: 4.00MB

OK (3 tests, 135 assertions)
HTML output was generated
http://php-apache-jenkins-php7-mysql5-5-6660/subdirectory/sites/simpletest/browser_output/Drupal_Tests_field_Functional_EntityReference_EntityReferenceAdminTest-1-90970228.html
(...etc...)

Remaining deprecation notices (2)

  2x: Passing an escaped locator to the named selector is deprecated as of 1.7 and will be removed in 2.0. Pass the raw value instead.
    2x in EntityReferenceAdminTest::testAvailableFormatters from Drupal\Tests\field\Functional\EntityReference

1.7 and 2.0 of what, you ask? Passing an escaped locator to what, and on what line? I grepped the codebase and found:
vendor/behat/mink/src/Selector/NamedSelector.php: 'Passing an escaped locator to the named selector is deprecated as of 1.7 and will be removed in 2.0.'

Which has:

    private function escapeLocator($locator)
    {
        // If the locator looks like an escaped one, don't escape it again for BC reasons.                                                                     
        if (
            preg_match('/^\'[^\']*+\'$/', $locator)
            || (false !== strpos($locator, '\'') && preg_match('/^"[^"]*+"$/', $locator))
            || ((8 < $length = strlen($locator)) && 'concat(' === substr($locator, 0, 7) && ')' === $locator[$length - 1])
        ) {
            @trigger_error(
                'Passing an escaped locator to the named selector is deprecated as of 1.7 and will be removed in 2.0.'
                .' Pass the raw value instead.',
                E_USER_DEPRECATED
            );

            return $locator;
        }

So it looks like somewhere in that test, a random string is somehow being passed to the above method, and if it matches the regex, BOOM, random fail. I read the test method though and the only variables in it are machine names, so that's as far as I got.

Proposed resolution

?

Remaining tasks

TBD

User interface changes

N/A

API changes

TBD

Data model changes

N/A

Release notes snippet