bsonrwtest is a separate package to facilitate use from multiple BSON-related packages in the Go driver. Once all of those packages are merged together, there is no longer a reason to have bsonrwtest separate or export any symbols at all.

Definition of done:

• Merge all code from the bsonrwtest package into test code in the bson package.
  Note that the bsonrwtest package only contains code used in tests. Once everything is merged into the bson package, all code in the bsonrwtest package can be added to a _test.go file so it's no longer exported.