Since the beginning,
install-wp-tests.sh (the setup script for plugin unit tests) has installed the WordPress testing framework from trunk. However, the WordPress project recently made changes to the testing framework which mean the trunk version is incompatible with older, tagged WordPress releases.
install-wp-tests.sh provided by WP-CLI to use an appropriately-tagged version of the WordPress testing framework. However, because
install-wp-tests.sh is added to a plugin during the scaffolding process, plugin authors will need to update the plugin copy of the script to fix failing tests. We weren’t able to get to the entire internet, but here are some plugins that have already been updated: pantheon-systems/wp-native-php-sessions, washingtonstateuniversity/WSUWP-Plugin-Color-Palette, ethymos/delibera
You can browse the full list of resolved issues on Github.