files from `inst/extdata` used in tests?

When running devtools::check, it is tried to install and test the package. When installing all subfolders under inst will be moved to the main directory. This alters the file structure and result in failing tests that are executed after the simualted installation.

How to handle this? The respective files (e.g. inst/extdata/ogs5_benchmarks and inst/extdata/ogs5_benchmarks_ref could be copied into the test dir, however also the executables, need to be copied there. In the end we do not want to ship ogs executables with the package.

I propose to add a default path to look for the ogs executables on the machine where the package is installed and executed. If the executables are found when running the tests, the respective benchmark tests will be executed, otherwise the benchmnark tests will be skipped.