You’re saying the test file’s location will differ depending on the test runner, so I assume it’s included in the project and copied together with the dll’s.
string path = AppDomain.CurrentDomain.BaseDirectory;
This will get you the folder where you’re executing the test from.
[Edit]
In Visual Studio.
Resharper -> Options -> Tools -> Unit Testing -> Run Results from: Specified Folder (or change the project output folder of your test project)
Where you can specify the folder of your test data, or relative to the specified folder.