Fix observeEvent stack trace stripping

[jcheng5]

Fixes #4162.

• Unit test

Explanation

Reactive objects have a lot of machinery that we generally want to hide, so they call ..stacktraceoff.. before they begin executing and ..stacktraceon.. right before they call user-provided code.

In the specific case of observeEvent, the inner observe() call had an explicit ..stacktraceon = FALSE. Once upon a time this was correct, as a line slightly preceding it took care of turning the stack trace back on. But during the introduction of bindEvent, that previous line went away.

[jcheng5]

I (lazily/expediently) added only a unit test for this specific issue. Ideally we'd have unit tests for every place in Shiny where we'd expect stack traces from:

1. Top-level app.R/ui.R/server.R/global.R
2. R/ subdirectory
3. UI code
4. UI function
5. Server function
6. Output renderers
7. observe
8. observeEvent (both eventExpr and handlerExpr)
9. observe |> bindEvent (both eventExpr and expr)
10. reactive
11. eventReactive
12. reactive |> bindEvent
13. Module UI
14. Module server function
15. Download handlers
16. session$registerDataObj? Or maybe not?
17. Session callbacks (onFlush, onFlushed, onEnded, onSessionEnded, etc.)
18. Bookmarking callbacks
19. renderPlot width/height functions

I'm probably missing more than a few...