Fail on missing results by default.

This PR causes tests to fail if their result files are not present.

To create the result files you can either:
 - Run `python setup.py test` with the `--create-missing`/`-m` argument.
 - Run a specific test module with the `--create-missing` argument. (NB. There is no `-m` version of the argument because of the possibility for confusion with Python's `-m` argument.) For example:
  - `python lib/iris/tests/unit/analysis/maths/test_add.py --create-missing`
  - `python -m iris.tests.unit.analysis.math.test_add --create-missing`