Taros and I run two testnets.
The first runs a duplicate of our current network software and configuration, and is used for developing new features, testing patches, and so forth for our network.
A second is used to run clean, unmodified InspIRCd and services software, with a configuration as closely resembling our live and test networks as possible, in order to reproduce bugs prior to reporting upstream (i.e. here).
At the current time, both run Anope 1.9.0p1 and InspIRCd 1.2 from git (actually, I don't remember if clean has services setup right now, but that's what it would be running as soon as we needed it). Clean runs straight from upstream git, while the normal testnet runs off our internal repository.
We'd probably install Anope 1.9.1 or 1.9.2 from git on either/both as soon as we wanted to begin working on either; we tend to do this for any software we want to test.
We don't stop regular users from joining or password it, but we don't advertise it either, although we don't particularly avoid mentioning it either. If people want to help play with our testing stuff, no reason not to let them. Neither have historically had oper access granted to people not on staff; we'd need to talk about giving it if someone asked.