This patch tidies how defaults are handled in the config, using a new
logic to allow "overriding" one config (the default) with another (the
user supplied).

It also improves how the comparisons are done in the tests, using the
more convenient "github.com/google/go-cmp/cmp" package, which also
prints nice diffs on errors.

This is in preparation for a future path where the override mechanism
will be reused.