More tests for @bind

@pankgeorg recently added a test for https://github.com/fonsp/Pluto.jl/issues/928, which is awesome(!) because we now have an end-to-end test for @bind.

This is a great start, and we should add some additional tests for @bind:

• Basic test: Move a slider and check if it triggers reactivity
• Debouncing: @bind x Slider(1:1000) and sleep(1); x. Moving the slider quickly from 1 to 1000 should not trigger at most 2 runs, not 1000.
• Are sliders synchronized within a browser session?
• (difficult) Are sliders synchronized between multiple browser sessions?
• https://github.com/fonsp/Pluto.jl/issues/1260
• https://github.com/fonsp/Pluto.jl/issues/1284
• https://github.com/fonsp/Pluto.jl/issues/1107