Qualified imports of pkg/testutil outnumber unqualified (55 to 7). Improve
consistency, and clarity, by changing the dot (unqualified) imports of
that package symbols to qualified.
This change addresses issues reported by the `staticcheck` tool and a
couple reported by the `golint` tool. It also adds missing issue links
to a couple of entries in the release notes. This change deliberately
does not address these warnings since it is unclear whether the project
owner would prefer to suppress or address them:
pkg/store/cmd.go:8:2: should not use dot imports (ST1001)
pkg/store/db_store.go:10:2: should not use dot imports (ST1001)
pkg/store/dir.go:8:2: should not use dot imports (ST1001)
