'Clear QT_HARFBUZZ before/after tests.'

@pytest.fixture(autouse=True)
def clear_harfbuzz(self):

    old_harfbuzz = os.environ.pop('QT_HARFBUZZ', None)
    (yield)
    if (old_harfbuzz is None):
        os.environ.pop('QT_HARFBUZZ', None)
    else:
        os.environ['QT_HARFBUZZ'] = old_harfbuzz
