We currently have test coverage for internal transactions in the following concurrency workloads:

• random_moveChunk_update_shard_key.js
  • This performs WouldChangeOwningShard update and findAndModify while migrating chunks. It uses both retryable writes and multi-document transactions.
• random_moveChunk_update_shard_key_kill_sessions.js
  • Same as random_moveChunk_update_shard_key.js but additionally kills sessions.
• random_internal_transactions_setFCV_operations.js
  • This performs WouldChangeOwningShard update and findAndModify while changing the FCV. 

The workloads provide test coverage for:

• Running an internal transaction on behalf of a client command running as a retryable write or multi-document transaction (has an lsid and a txnNumber).
• Retrying and reconstructing responses from WouldChangeOwningShard noop entries.
• Killing internal sessions for retryable write.
• Downgrading FCV when there are existing and active internal transactions for retryable writes.
• Migrating chunks when there are existing and active internal transactions for retryable writes.

We still don't have concurrency test coverage for the following:

1. Running an internal transaction on behalf of a client command not running in a session (no lsid) or not running as a retryable write or multi-document transaction (no txnNumber)
2. Retrying and reconstructing responses from retryable insert, update, delete and findAndModify (with a pre or post image) entries.
3. Killing internal sessions for non-retryable writes.
4. Reaping internal sessions for retryable writes and for non-retryable writes. 
5. Downgrading FCV when there are existing or active internal transactions for non-retryable writes.
6. Migrating chunks when there are existing and active internal transactions for non-retryable writes.
7. Resharding the collection when there are existing and active internal transactions for retryable writes and for non-retryable writes.

Note that we have test coverage for (2), (4), (6) and (7) in jstests/sharding/internal_txns.