Install mysqli in test 11.3 database fixture

Problem/Motivation

The upgrade path tests in mysqli jobs fail when the mysqli module is not installed. Since mysqli jobs aren't typically run until the commit pipelines, this can go uncaught during MR review.

There's a test fixture that installs mysqli on top of the dump, but people need to remember to add it manually to tests, which is not obvious to do.

From #3580877-41: Remove updates added prior to 11.3.0 from 12.x

I think we'd be better off overall if we could get mysqli enabled in the original database dump rather than an extra fixture, but this is fine as a hotfix.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet