Factor out a common ChainDbArgs from our test suites

[dnadales]

This task is a matter of auditing/cataloging how each test that creates a ChainDbArgs sets the various arguments. Most tests don’t care about most of those arguments, so this task is to identify which arguments can have a default value and consolidate those in a re-used definition.

Why?

This will help maintain and add new ChainDB tests.

Acceptance criteria

Existing constructions of ChainDbArgs in various tests are simplified by reusing the common component.