add: verifyInputMessage for mock-session

[stefanedwards]

"Proof of concept"

Added methods to mock-session for verifying that specific calls to session$sendInputMessage were performed. Note: Before merging, these methods should be expanded to include session$sendCustomMessage and session$sendBinaryMessage!

This suggestion is inspired by how mocked classes in C# with Moq behave; that the mock itself is able to verify whether a given call was performed. By allowing mock-session to verify (upon request, naturally) it makes it easier to write unit tests of Shiny update methods, as the developer doesn't have to write boilerplate code. See e.g. following examples from tests/testthat/test-update-input.R:

Before:

createModuleSession <- function(moduleId) {
    session <- as.environment(list(
      ns = NS(moduleId),
      sendInputMessage = function(inputId, message) {
        session$lastInputMessage = list(id = inputId, message = message)
      }
    ))
    class(session) <- "ShinySession"
    session
  }

  sessA <- createModuleSession("modA")

  updateRadioButtons(sessA, "test1", label = "Label", choices = letters[1:5])
  resultA <- sessA$lastInputMessage

  expect_equal("test1", resultA$id)
  expect_equal("Label", resultA$message$label)
  expect_equal("a", resultA$message$value)
  expect_true(grepl('"modA-test1"', resultA$message$options))
  expect_false(grepl('"test1"', resultA$message$options))

After:

  session <- MockShinySession$new()
  updateRadioButtons(session, "test1", label = "Label", choices = letters[1:5])
  session$verifyInputMessage("test1",
    expect_equal(.$label, "Label"),
    expect_equal(.$value, "a"),
    expect_true(grepl('"mock-session-test1"', .$options)),
    !expect_false(grepl('"test1"', .$options)) ## negate returned FALSE from expect_false
  )

The verifyInputMessage has built-in support for the testthat-package, without depending on it. Calling verifyInputMessage is enough in a test_that:
Before:

test_that("session supports sendInputMessage", {
  session <- MockShinySession$new()
  session$sendInputMessage(inputId=1, message=2)
  expect_true(TRUE) # testthat insists that every test must have an expectation
})

After (with actual testing):

test_that("session supports sendInputMessage", {
  session <- MockShinySession$new()
  session$sendInputMessage(inputId=1, message=2)
  session$verifyInputMessage(1, . == 2)
})

As noted initially, these methods should be implemented for sendCustomMessage and sendBinaryMessage before including, and I will gladly do this (letting the three methods use the same routine, somehow).

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