Tie Quality and Testing sections together more clearly

These two pieces of guidance are very closely related but weren't explicitly
connected. I've moved some of the testing content to the quality section and
added some links.
Pretty sure this was meant to be 'automated' not 'agile'

This sentence makes more sense if it's talking about 'automated' testing,
not 'agile' testing. The mention of types of tests then makes more sense
in the code testing section.
Add section on 'approaches' to cover test-first and bdd

We had some confusing content elsewhere in the guidance which referred to BDD vs
Acceptance Testing, but we never explained BDD.
Link to Martin Fowler's explanation of Ubiquitous Language

As @jabley pointed out, this could do with a bit more explanation and Martin Fowler'
s piece seems the most straightforward source.