test/config: add several application script tests

The declarative config has the `app` section with following three
parameters:

* `app.file` -- the path to the script file
* `app.module` -- the same, but the script is searched using the Lua
  search paths (and loaded using `require`)
* `app.cfg` -- a user provided configuration (arbitrary map)

This commit adds a few success/failure cases. The goal is to have simple
test examples to add more ones easier in a future.

Implemented several test helpers for typical scenarios that can be used
outside of the application script test.

Part of #8862

NO_DOC=testing improvements
NO_CHANGELOG=see NO_DOC