p-trap installation by newbie: cement cure time
It's probably fine. ABS cement hardens in seconds -- probably in less than 20 seconds the joint would be locked up enough that it would not be easily moved by hand.
A joint won't be cured enough to withstand any meaningful amount of water pressure that fast -- a plumber would probably get an unhappy surprise if he cemented test plugs or fittings into place and immediately proceeded to fill a 20-30 foot tall column of water to test a new home's multi-story drain system.
In your case though the pressure would not have exceeded about 1 inch water column. If it didn't leak in your first test, go ahead and test it again. If it continues to not leak that's a good sign that it worked out well enough.