tests: move TemporaryDirectoryTestCase to util module
tests: move responses_add_mocks to tests.util
dataset: support configurable tile resolution

This is a requirement for being able to test the dataset code with
non-full resolution tiles.
test: change mock data tile size to 20x10

The input tiles are non-square and non-power-of-two in size. Reflect
this in the mock data.

Update the generate-dem-chunks.sh script to also save a ruaumoko-format
dataset for use in later testing.
tests:dataset: add stub tests for Dataset

These tests do little more than check that the dataset can be opened and
that the mock dataset location exists.
Merge pull request #20 from rjw57/stub-tests-for-dataset

some stub unit tests for Dataset