hijack dub test

[Nicholas Wilson via Digitalmars-d-learn]

My project is a library, but I also need to test it and unit 
tests won't cut it (external hardware).

How do you set up the dub.json to build the library normally but 
when it is invoked with `dub test` it runs a separate 
configuration that also includes files in the `source/test` 
folder, but are excluded when not testing.

If thats not possible how does one specify that `source/test` 
should not be built when building a library and only 
`source/test` should be built when doing tests, but the tester 
depend in the library?

Thanks
Nic