docs(testServer): Update example to simulate button click

[gadenbuie]

Fixes #2745 by updating the example in ?testServer to simulate a button click. #2745 points out that unlike other inputs, it's not obvious how to simulate the update for a button click.

I modified the existing example to follow a more real-world example, albeit a very simple one. I included the UI function for the module to help make the example feel more realistic – the current example demonstrates server logic testing but doesn't map well to a real app.