Create test-friendly `ResourceDirectory` to retrieve resource files

[stijnmoreels]

Is your feature related to a problem? Please describe.
When retrieving resource files in a test project, one has to go through some loops to first get an existing directory/file and then get the right file. There is also a problem with hard-coded file names, which makes it harder to rename resource files.

What solution do you propose?
Create a test-friendly ResourceDirectory alternative that can be navigated (should be immutable) to a sub-directory and retrieve files by extension or part of a file name. All operations should come with a test-friendly exception message.