[nwtv2003]

I'm pretty sure Nickelodeon time shared with some other service or two before Paramount came along, and IIRC that was right up to the back end of Nick's hours either side, so in other words there wouldn't be any time for test cards even if they did have one to use in the analogue days.

Did they not share with the Performance Channel at one point? At least on some systems.

They did on my local cable, when it was Nynex, but I think it was more the fact they didn’t have an agreement to carry Paramount until September 1997. Essentially the final minute of Nickelodeon were promos for Paramount, before switching the feee to Performance. This stopped when Paramount became available, and in their wisdom Performance Channel started to timeshare with TV5: 

youtu.be 

They had other crazy cable only timeshares such as God Channel with Eurosport, Granada Breeze and National Geographic, before switching to share with MUTV. I think it was just a cable thing to maximise space. 

I don’t recall seeing a Nickelodeon testcard when they shared with Performance, more than likely they would have used the Channel Guide to fill the downtime like they did with a lot channels.