runTests() doesn't source app.R before running tests

[alandipert]

I noticed while working on rstudio/shinycoreci-apps#2 that shiny::runTests() doesn't source app.R before sourcing files in tests/.

It only sources files in R/ (if there are any), globals.R and server.R, depending on the value of shiny.autoload.r.

This is problematic if your app consists only of an app.R, but your app.R contains functions (or module functions) that you wish to test.

I think we should source app.R after any files in R/, but I'm open to suggestions about how we should handle the presence of app.R combined with the presence of server.R/globals.R. I figure at a minimum we should signal a warning.