Update test suite to run on MySQL 8

[watson]

MySQL 8 was just released and our test suite doesn't seem to run on that version.

Here's an example of the failure: https://apm-ci.elastic.co/job/elastic+apm-agent-nodejs+pull-request+multijob-run-tests/435/NODEJS_VERSION=8,label=linux/console

The issue is most like this one: mysqljs/mysql#1507

Todo

• [ ] Update test suite to run on MySQL 8
• [ ] Update docker-compose.yml to use MySQL 8

[watson]

Consider using :latest instead of :8