Writing unit tests for ASP.NET Web forms application is NOT common and very hard to accomplish. However, it does not mean that the project should skip it.
Unit tests are corner stones
of mission critical applications and they provide reliability and peace of mind that core functionality performs as expected.
Actually you may introduce unit testing much easier with a ASP.NET MVP pattern that could be used in your web forms development. It will introduce separation of concerns and ability to write essential unit tests
.
Some references that might be helpful to look are: