After SERVER-111031, default write concern ({w: "majority"}) is now applied to shardsvr nodes. This causes forceSyncSource() in sync_source.js to hang indefinitely when called on a replica set where another secondary is already blocked by a failpoint.

forceSyncSource() temporarily blocks its target node with stopReplProducer before inserting a document. If the caller has already blocked a different secondary, then 2 of 3 nodes cannot replicate, making majority impossible.

We should use explicit {w: 1} for the insert in forceSyncSource(), matching the pattern already used in assertSyncSourceChangesTo() in the same file.