[Fuego] location for non-upsteam tests (prep for pre-built docker image)

Hello!

I was curious if you think it would be a nice feature to allow users of
Fuego to use a separate location for their custom tests and plans.

I am asking because I am working on a couple of tests that will not be
published to upstream and I'd like to have the possibility to bind-mount
all my custom things into the container and Fuego picking these up.

Background: Aside from having a location that is distinct from official
Fuego tests I would like to have such feature as I am really eager on
making it possible to have a pre-built Fuego docker image. Then the user
would decide if she/he wants to add additional tests using a bind mount.


Best,

Lazlo