SQL Express connection string: mdf file location relative to application location

I am using SQL Express databases as part of a unit test project in c#. My databases is located here:

./Databases/MyUnitTestDB.mdf

I would like to use a relative path or variable in the app.config rather than having my connection string defined as:

AttachDbFilename=C:\blah\blah\blah\yea\yea\yea\MyApplication\Databases\MyUnitTestDB.mdf 

I have seen the use of |DataDirectory| but am I correct in thinking this is only applicable to web applications?

I want to control this in the application configuration file, as in production the application uses a hosted sql database.