Maybe this does what you want?

DynamicModule[{tmp},
  Row[{ "tst1"
    , Spacer[4]
    , Slider[
        Dynamic[tmp
          , (tmp = #; Echo[tmp]) &
         ]
        , {1, 5, 1}
        , Appearance -> "Labeled", ImageSize -> "Small"]
      }
  ]
]

I have found that I can reduce (not eliminate) the need to debug my functions in this way by using unit tests as part of a more test driven approach.

Mathematica supports test driven approaches using a "Testing Notebook" under:

File | New | Programmatic Notebook

If you would rather keep your unit tests closer to your code, you can include VerificationTest

In addition to unit tests, I also find asserts and checks useful.

Thanks for the great question.