Testing if the module has been called in shiny

I write some shiny tests recently using testServer which is great in most of the scenarios. I'm thinking about following problem which I can't solve now.

Let's imagine simple modules (ui/server)

child_ui <- function(id) {
  ns <- NS(id)
  div(
    selectInput(inputId = ns("select"), label = "select", choices = c("a", "b"), selected = "a"),
    verbatimTextOutput(outputId = ns("out"))
  )

}
child_srv <- function(id) {
  moduleServer(id, function(input, output, session) {
    output$out <- renderPrint(input$select)
  })
}

... which are called inside of the other module but through the specific function call_child.

parent_ui <- function(id, module) {
  ns <- NS(id)
  div(
    module(ns("parent"))
  )
}

parent_srv <- function(id, module) {
  moduleServer(id, function(input, output, session) {
    call_child <- function(module) {
      module(id = "parent")
      NULL
    }
    call_child(module)
  })
}

What I would like to do is to check whether child_srv has been called and I wonder if there is some way to deduct this. Preferred solution is to keep call_child untouched (maybe some evidences are in session object).

The only thing I can do is following:

testServer(
  app = parent_srv,
  args = list(module = child_srv), 
  expr = {
    tinytest::expect_null(call_child(module = module))
})

# example app
shinyApp(
  ui = parent_ui(id = "root", module = child_ui),
  server = function(input, output, session) 
    parent_srv(id = "root", module = child_srv) 
)

Does anyone has any advice how to properly test this scenario?