Split testing module

[standage]

For my Sept 2017 workshop, I split the existing testing module into two. In the morning we covered the fundamentals of testing, and then in the afternoon we covered test-driven development. I think this level of granularity is appropriate and useful in adapting the modules for a particular audience.

I will fix the sequencing of these modules once #51 is merged.

[gvwilson]

Merging - I'll fix the sequencing.