The consistency checks performed inside ReplSetTest stopSet() do not pass explicit read/write concern (RWC), instead assuming that the implicit server defaults will be applied (which was true historically). However, this is not the case if the RWC defaults have been applied to the ReplSetTest inside the jstest, leading to failures and/or incorrect semantics (ie. potentially flawed checks). Therefore the server operations performed by these consistency checks should pass explicit RWC corresponding to the implicit server defaults of {w: 1} and {level: "local"}, where those operations permit RWC.

Original summary

checkReplicaSet should set an explicit writeConcern when running collMod

Original description:

As part of the repl consistency checks in replsettest.js, we run a collMod (to wait for any index builds that may be in progress). That command is run without a writeConcern set, so it will inherit the default WC, which may have been changed during the execution of a test. We should pass in a WC explicitly so it doesn't depend on that.