PHP 5.5: ApcuBackendUnitTest fails with bizarre error

[wimleers]

Command:

testbot@lubuntu:~/drupalci_testbot$ sudo DCI_DBTYPE='mysql' DCI_DBVER='5.5' DCI_PHPVERSION='5.5' DCI_TESTGROUPS='--all' DCI_DRUPALBRANCH='8.0.x' DCI_CONCURRENCY='16' DCI_INSTALLER="drush" ./containers/web/run.sh

Relevant portion of output:

Drupal\system\Tests\Cache\ApcuBackendUnitTest                  0 passes                                      

FATAL Drupal\system\Tests\Cache\ApcuBackendUnitTest: Found no database prefix for test ID 69. (Check whether setUp() is invoked correctly.)

Code in Drupal 8's run-tests.sh that triggers this:

function simpletest_script_cleanup($test_id, $test_class, $exitcode) {
  // Retrieve the last database prefix used for testing.
  list($db_prefix, ) = simpletest_last_test_get($test_id);

  // If no database prefix was found, then the test was not set up correctly.
  if (empty($db_prefix)) {
    echo "\nFATAL $test_class: Found no database prefix for test ID $test_id. (Check whether setUp() is invoked correctly.)";
    return;
  }

  …

Notably, not a single other test (so far, it's still running, will update this once it's done) has this failure.

[wimleers]

Now it no longer fails, hurray!