The /transactions/write-concern test is failing in master after a bug with parsing the runOn constraint of the spec tests was fixed in CDRIVER-3266.

The failing test was using an unacknowledged write concern as a collection option, and expecting operations within the transaction to not apply that write concern and succeed:

[2019/08/02 14:22:31.344] Expected success, got error "Cannot use client session with unacknowledged writes":
[2019/08/02 14:22:31.344] { "name" : "insertOne", "object" : "collection", "collectionOptions" : { "writeConcern" : { "w" : 0 } }, "arguments" : { "session" : "session0", "document" : { "_id" : 1 } }, "result" : { "insertedId" : 1 } }

Though the transaction's spec states:

Drivers MUST NOT add the transaction’s writeConcern or any writeConcern inherited from the collection, database, or client to any preceding commands in the transaction.