Test the indexing of concurrent specimen uploads

[alex-hancock]

There are multiple examples of two distinct bundles containing data for the same specimen. If two of these bundles are submitted concurrently, we want to ensure that Elasticsearch properly processes the two uploads.

This test fails if we see only one document indexed in the process (as its write operation is only on the data it read prior to the first operation's write).
This test succeeds if we see a log message that one of the processes retries and merges the document properly.