The mongos pinning prose tests were introduced in 08c94e6 for SPEC-1140. A few months later, 884d364 (SPEC-1168) added test syntax to assert that sessions were unpinned.

Given that we have spec test syntax, do the prose tests still provide any value? One of these executes 50 transactions to assert that more than a single mongos server is used over the course of the test, which seems like an expensive test worth removing if it's no longer needed.