Re-enable the tests under SwiftPM.

[samdeane]

Since we can't use a bundle to find the test data, we fall back on a relative path from the built executable.

This is a bit skanky, but it's a useable workaround, and better than not having the tests at all.

[kovpas]

Thanks! Could you please move this to a separate method, like you did in #14?

[samdeane]

Yeah, no problem. I should have done it in the first place :)