Do not use app object when testing a package

[schloerke]

From #307 (comment)

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.

[schloerke]

This may go away if we can use testthat file loading

[schloerke]

With v0.5.0 via #328...

Ideally, a file location is the best.

When customization can be used, a package function can be supplied. driver <- AppDriver$new(run_app). This works as a regular function may be given. However, a library(mypkg) is required as it is run in the background R process. Behind the scenes, local. package functions are upgraded from run_app to function() { library(mypkg); run_app() }. When calling library(mypkg) when locally testing {mypkg}, pkgload::load_all() will be called upon the package path.