Following has been observed for the first time when phpunit is run. Every consecutive run passes.

1) core_cache_config_writer_testcase::test_add_edit_delete_plugin_instance

Failed asserting that an array has the key 'addplugintest'.

/var/www/html/moodle/cache/tests/config_writer_test.php:181

/var/www/html/moodle/lib/phpunit/classes/advanced_testcase.php:80

To re-run:

 vendor/bin/phpunit core_cache_config_writer_testcase cache/tests/config_writer_test.php

Steps to reproduce, with PHP 7.1

1. php admin/tool/phpunit/cli/init.php
2. vendor/bin/phpunit core_cache_config_writer_testcase cache/tests/config_writer_test.php
3. ^ Fails.
4. vendor/bin/phpunit core_cache_config_writer_testcase cache/tests/config_writer_test.php
5. ^ Now passes.
6. php admin/tool/phpunit/cli/util.php --drop
7. php admin/tool/phpunit/cli/init.php
8. vendor/bin/phpunit core_cache_config_writer_testcase cache/tests/config_writer_test.php
9. ^ Fails.