The two following lines are flacky underload, yet the final result is correct.
The command involves background pre-check of output, these are not stable
probably because they run in parallel in multiple process.
I spent a couple of hours trying to understand the pattern and gave up. The
documented intend of these tests is safely guaranteed by checking the cache
content after the command.
If it become useful to start testing precise internal details of the, they will
have to be tested in a more appropriate framework than .t tests.