The failure is in ‘process-tests/sentinel-with-multiple-processes’
and happens on all the configurations being tested. It happens in
both ‘make check’ and running the test individually. It can be
reproduced using a script like:

  #!/usr/bin/env bash
  echo OK >> ~/tmp/process-tests.txt
  cd ~/src/emacs
  rm -f test/src/process-tests.log
  make test/src/process-tests.log

and the command ‘watch -e script.sh’. For an idea how often it
fails here are the results of five runs of the above on two
different machines:

  wc -l process-tests-*.txt
    157 process-tests-1.txt
    259 process-tests-2.txt
    329 process-tests-3.txt
    112 process-tests-4.txt
     96 process-tests-5.txt
    950 total

  wc -l process-tests-*.txt
     11 process-tests-1.txt
     14 process-tests-2.txt
      9 process-tests-3.txt
     11 process-tests-4.txt
     11 process-tests-5.txt
     56 total