We should have a RAII type that allows for enabling or disabling a feature flag during execution of a unit test, or part of a unit test.

While in general unit tests should not rely on environmental settings such as command line arguments or setParameter values, some of our C++ unit tests for high level sharding or replication behavior do in fact depend on subsystems involving such parameters. Using an RAII type allows specific unit tests to select required behavior or even iterate over multiple settings where appropriate, while still maintaining the property that the unit test itself does not depend on command line arguments or similar.