Use len(buffers) rather than fixed ints in tests
Use len(buffers) rather than fixed ints in tests

So that the tests still pass if we adjust the number of buffers and the
numbers seem a bit less magic to casual readers of the tests.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf