Right now, Redshirt has the account shared bank up for testing, which goes live this week. Tribble, being for season 6, is pretty sparse. There's a couple queue events are there right now, but there's rarely enough people to get the queue to pop. So if you actually want to test something, Redshirt.
Judging from S3-5, the test population will grow as more stuff (like starbases) are added, but the real boom will be after the test rewards are revealed. If it's another bonanaza of free crap like S5, testing will boom enough that holodeck will get sparse. If it's a more typical test weekend, there'll be a lot of people idling for a few days one weekend.
Not quite what I meant. Right now, if you go, the two things up for testing won't even be accessible because population's been too low to get 5 people queued at once for them. When more S6 features are out (watch the season 6 dev blog) for testing, more people will be on tribble and you'll probably be able to get somewhere. When the S6 rewards are announced, populations will jump, but it depends on how the rewards are structured this times whether that'll mean all those people will actually be doing useful testing.