Have _test.go files use the dynamodb package

Putting them in a distinct dynamodb_test packages requires all the
import paths to be updated if you are working on a fork.
Merge remote-tracking branch 'origin/tests-package-refactor' into entity-support
Revert "Support only dynamo-encoding the 'Item' field"

This reverts commit 028c93a2783de487bea58e531dd34999f140e939.
Wire up the PutEntity method