Directory creation in io unit tests not thread safe

[Prof Garth Wells]

The creation (and later) delete of output directories in the IO unit tests is not thread-safe, i.e. can cause pytest to fail when run in parallel using xdist.

[Jan Blechta]

Is it a code in dolfin_utils.test.fixtures._create_tempdir? That might need some careful rewrite with thought about xdist parallelism.

[Prof Garth Wells]

I made some fixes for this - adds pytest-xdist thread index to the directory name.

[Jan Blechta]

Perfect.