is writing unit tests for ASP.NET webforms something that is done often, and if it is: how do i resolve the "dynamic environment" issue ?
It is not often done. Working with UI elements isn't what unit tests are good at, since there's no great way to programmatically verify that the right things end up on the screen at the right places.
Would you guys advise me to start writing unit tests ? I think it might help me in the actual implementation, but then again i feel like it might slow me down.
Where applicable, yes. They can be very helpful in verifying specific cases in a repeatable manner. They help act as 'friction' against troublesome changes, and as a failsafe when you're making better changes.
One thing to note is that they will usually slow you down.
This is okay.
Spending a little time now will (if done well) save you time in the future because they catch some bugs, prevent recurrance of some bugs, and allow you to be somewhat more comfortable doing other improvements in the code to keep it from rotting.