Provide function to get path to test dir

[hadley]

So you can write code that can be run in tests and interactively.

[jennybc]

Related: nice solution for sourcing all helper- files for interactive test development? A separate issue?

[hadley]

@jennybc source_test_helpers()?

[jennybc]

I sure wish I had known about that sooner. I guess the function described here will make a nice default for the path.