Stop using 'iris.tests'

[pp-mo]

We don't really want to install requests + filelock. which are now required to use iris.tests.
(cf problem here)
All we actually use it for is to locate the installed test data.
Which requires setting OVERRIDE_TEST_DATA_REPOSITORY and then using iris.tests.get_data_path.

So we might as well implement our own mechanism for accessing the iris test data
(but could still use the same OVERRIDE key)