Some versions of tor have bugs that chutney needs to work around, for example, legacy/trac#20996 requires tor versions <= 0.3.0.1-alpha to set `UseMicrodescriptors 0` when setting `ClientUseIPv4 0`. But we want to test newer versions without this workaround, so we need some kind of versioning feature. (Or test alternatives, where we report more than just pass or failure - but I think that's a different ticket.)