feat: merge DB

[ElPicador]

This is a very draft PR to answer this need.

I had a hard time understanding how testmon works internally, and also how to test this.

I'll happily get any feedback, so we can agree and merge this in testmon.

It would be very helpful for us as we have multiple workers in our CI that run tests, and we would need to merge testmon result so it can be reused later