It is common to run the tests on very loaded machine when concurrent run might
take a bit longer. Such message are usually harmless, but anoying as they break
Test that explicitly depends on this value have been adjusted. This make them
more robust anyway.
A fun case was test-clone-pull-corruption.t which, without the previous
changeset introducing extra flushing, ended use having a line 31 (`pulling from
../source`) changing order because the warning message was no longer flushing
stdin before using stderr (stderr being invisible in the test).