config: support DATABASE_URL when testing

[danp]

Describe your changes

I run postgres locally but without postgis. It would be nice to run the postgis docker image on another port/etc and point to that.

To help that, support DATABASE_URL when testing. If not set the database config is unchanged.

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [ ] If it is a core feature, I have added tests.
• [ ] Are there other PRs or Issues that I should link to here?
• [ ] Will this be part of a product update? If yes, please write one phrase about this update in the description above.