What does it mean when I run MediaWiki unit tests on my server, and phpunit skips some tests with this error:

Extension mysql is required

This is on a fully working MediaWiki server with mySQL installed, running on Ubuntu 16.04.

$ mysql --version mysql Ver 14.14 Distrib 5.7.12, for Linux (x86_64) using EditLine wrapper

$ dpkg -s php-mysql Package: php-mysql Status: install ok installed ...

Thank you, DanB