The offer tests contained a stray test case that I suppose was once
intended as a base class for all the other tests. But it's only used
twice, and for very little gain, so let's delete it.

Hmmm, this turned out to be a monster commit.  This change allows the
basket to be able to correctly calculate prices including tax.

It also requires a whole load of test changes since all baskets now
require a strategy instance to be assigned.

Now using --spec format which is more readable (most of the time).

They weren't applying to lines properly.  Fixed with test.