unittest which uses a disk file

[Victor Porton via Digitalmars-d-learn]

What is the rule for unittest which uses a file (containing 
example data for testing) available only in the source 
distribution, not in binary distribution?

I am writing a library.
The library has also a file main.d which is compiled only in DUB 
"application" configuration (I use this configuration solely for 
testing.)

Maybe I should put unittest { } into main.d not in the module 
which I test?

Also, what is the correct way to locate the file in the 
filesystem?