Load global vars when saving app objects

[schloerke]

Fixes #303
Fixes #295

• Try using global env when assigning
  • Ans: Shimming an environment between the server and global env
  • Ans: Using global env. Performing multi asign using list2env(envir=globalenv())
• Try using {future} / callr to pass app object instead of saving to temp arg
  • Ans: While {future} does a great job at capturing what is needed, it does not work well with stdout/stderr. This is a deal breaker.

[jcheng5]

Am I correct that the main three cases we care about are:

1. Writing tests for a nontrivial app that is in an app.R
2. Writing tests for a nontrivial app that is using golem/rhino(?)
3. Writing tests for trivial apps that exercise some feature or component that is not specific to a single app

The kind of cross-process de/serialization that this PR is fixing, totally makes sense to me for case 3. It makes me a lot less comfortable if people are relying on it for case 2. I'd love to see us make a formal recommendation for using shinytest2 with golem/rhino that doesn't involve serializing any code objects.

[daattali]

The biggest benefit that I personally wanted from this is to be able to test a shiny module. If there is another good way to test a shiny module, please let me know.

library(shiny)

mod_ui <- function(id) {
  ns <- NS(id)
  textOutput(ns("foo"))
}

mod_server <- function(id) {
  moduleServer(
    id,
    function(input, output, session) {
      output$foo <- renderText("bar")
    }
  )
}

app <- shinyApp(
  mod_ui("test"), 
  function(input, output, session) { mod_server("test") }
)

driver <- shinytest2::AppDriver$new(app)
driver$get_logs()  #  Error in mod_server("test") : could not find function "mod_server"

[daattali]

This PR does indeed fix my usecase in #303