add github action for mysql backend

[4e554c4c]

Currently the MySQL backend is not tested, as it requires MySQL server
to run. I have added an integration test to the GitHub workflows which
starts MySQL server in a container, which allows the backend to be
tested. This ignores several "issues" that exist with the tests, such as
the fact that the test is non-idempotent, and fails to run twice.

[4e554c4c]

reasons for the PORT environment variable:
you cannot use port: 3306:3306 because GitHub already runs mysql (v5.7) on github runners. An alternative to this is somehow stopping the host mysql before the container starts?
Really GitHub can assign the port for you, and you can somehow grab the port out of the job dictionary, but I cannot figure out the syntax. See here: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idservicesservice_idports

reasons for the MYSQL_PWD environment variable:
mysql and mysqladmin will yell at you if you specify the password on the command line and then not work. you can specify it in an option file, but the option file must be created with mysql_config_editor, which also doesn't let you specify the password on the command line. The MySQL docs state

Use of MYSQL_PWD to specify a MySQL password must be considered extremely insecure and should not be used

but at least they let us use it unlike the --password= command line option.

[jessepeterson]

lgtm! thanks!