Add a way to stop the session fully at the end of a scenario

[stof]

In some cases, a scneario may end up in a state where Mink will not be able to reset the session enough (see minkphp/Mink#642 for such report). The @insulated tag covers the case where the scenario itself requires full isolation, but we would need a different tag for the case where the scenario requires a full restart at the end because of the final state.