For reasons not entirely clear, stdout will get some data injected when
pytest-cov is running. Our tests that
check for clean stdout need to ignore this.
We check for an environment variable that is defined only when coverage is
running.
Due to difficulties of getting this to work on Python 3.8, Windows, and
high probability that this behavior is now gone from Tesseract 4.0+.
Originally added in 2017.