Minor corrections to file_is_newer_than tests.

This PR makes minor corrections/modifications to the unit test file for `iris.util.file_is_newer_than`:

* copyright date is corrected (the file was created this year)
* sphinx references use the correct type (function not method)
* runs `iris.tests.main()` instead of `unittest.main()` if the module is executed

All in all these are rather trivial changes but we might as well strive for high quality right from the start whilst the unit test area is still rather unpopulated.