If a Skarner grabs an enemy and runs to the center of an allied Janna ult (which usually pushes enemies away), will it cancel Skarner's ult and push the enemy away or will Skarner still have a solid grab onto the enemy and janna will push everyone else away?
No it will not, Scarner will be pushed together with his target but I didn't find any videos that shows that specifically. But I found this video, where a blitz pull a Scarner and his target together (even with another blitz pulling the Scarner target at same time).
In this video, you can see that Scarner ultimate, overlay WW
But specifically for Janna ultimate, I could not find any videos or references on the web.
You can try submit for them, so they can test:
Or you can ask for someone in the Chat-room, so they can help you to test it in a custom game.
Hope I could help in something.