Add a note to the "Contributing" page to suggest enable concurrency when running the tests locally.

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diffoscope's test suite relies on `pytest <https://docs.pytest.org/>`__;

to run all tests use ``pytest[-3]``. For faster interactive development

here's an example of how to run a (much) smaller subset of tests:

to run all tests use ``pytest[-3]``, appending ``-n 4`` or similar to enable

running tests concurrently. For faster interactive development here's an

example of how to run a (much) smaller subset of tests:

    pytest  -v --exitfirst -k lib tests/comparators/test_elf.pyc [ --pdb ]