Instead of batching based on the Resource pointer which is not guaranteed
to uniquely identify common Resources, use the `String` method which is.
Renames the external test package to be identified as the integration
testing. This testing structure (the mock exporter) batches exported
resources transmitted from the resource. This is needed as there is no
guarantees that a batch will be exported in a single payload, therefore,
it is not an ideal place to add resource batch testing for the exporter
itself.
Add unit tests to ensure the Exporter is correctly batching spans by
resource.
