Fix CounterButton initial value

[yha]

Fixes a double update when a cell defining CounterButton refreshes. I also changed the tests to catch this kind of problem, but I'm not sure if that's the right fix. Pluto ignores initial_value if there is a Base.get, but the docs for initial_value seem to say it shouldn't. So maybe this should be fixed in Pluto instead?

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

  julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/yha/PlutoUI.jl", rev="counter-button-init")
julia> using PlutoUI

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