Add ability to download geth logs

[emizzle]

Since there is an upcoming refactor that will automatically download geth logs on test failure, this can be disregarded if the changes are better suited for the refactor. However, I needed these logs badly to understand what is happening in the marketplace interactions, so figured I'd open a PR and possibly help anyone else in the same boat.

[benbierens]

I like this imporvement, but I propose to not merge it. The plugin-refactor makes this possible out-of-the-box.