Add tests for atomic_write function

[lucc]

This is a followup to #343.

The original tests for the atomic_write function are in the atomicwrites repository: https://github.com/untitaker/python-atomicwrites/blob/4183999d9b7e81af85dee070d5311299bdf5164c/tests/test_atomicwrites.py

They were written for pytest but khard uses unittest from the stdlib, so I migrated them here.

I would like to include them in khard under the existing GPL license in order to keep the license situation in khard simple (= one license). But the original code is licensed by the MIT license in the atomicwrites project.

Hence the question for @untitaker: Is it ok if we re-license these migrated tests under the GPL license?