Should have function to wait for input to be a certain value

[wch]

For example, this waits until input$state is a value other than NULL or "".

for (i in 1:100) {
  state <- try(app$getValue("state", "input"))
  if (inherits(state, "try-error") || identical(state, "")) {
    Sys.sleep(0.2)
  } else {
    break
  }
}

As seen in https://github.com/rstudio/shinycoreci-apps/blob/73997616e9d2f6584cded524ad46faf44154d4e2/021-selectize-plot/tests/shinytests/mytest.R

This should also be documented on the website, possibly in the FAQ.

Also, we might need to check how long getValue will block for before it times out. Ideally, we'd be able to call it with a short timeout.