The collection_fragmentation.js test fails on the concurrency embedded router suites with both multi statement transaction and balancer. The test doesn't run at all on variants without balancer enabled, but it does work on variants without multi-statement transactions enabled. The failure pattern is that all test threads get stuck in an infinite loop of failing to create an index inside a transaction because the readConcern is set to snapshot, and then the transaction being retried because committing it fails. Note that changing the readConcern to local just results in a different failure where createIndexes is prohibited in transactions on pre-existing collections.

Investigate why this issue is occurring on embedded router suites and either

1. resolve the issue, or
2. make a determination that the test is incompatible with embedded router mode and create a corresponding exclusion with documentation.