Add noDb arg to testbot

[Powersource]

Or maybe an noUse which doesn't secretstack.use anything. Or is the db the only thing being .used and they'd be the same thing?

Followup from #96 (comment)

The testbot is generally useful but we want easier/more control over what modules it imports, and we don't want this hack where we end up importing db1 even if we don't use it.

• Add noUse opt scuttle-testbot#26
• Don't import db1 #105