exec_prerun pollutes JSON output

[jfpanisset]

Please acknowledge the following before creating a ticket

• [x] I have read the GitHub issues section of REPORTING-BUGS.

Description of the bug: exec_prerun pollutes JSON output

Environment: CentOS 7.6 and 8.2

fio version: 3.28

Reproduction steps In a job file that uses:

exec_prerun=/path/to/script.py

if you specify -output-format=json and -output=foo.json, you end up with non-JSON data at the start of your output file:

<server01> jobtype1 : Saving output of prerun in jobtype1.prerun.txt
<server01> jobtype2 : Saving output of prerun in jobtype2.prerun.txt
<server01> jobtype2 : Saving output of prerun in jobtype2.prerun.txt
<server01> jobtype1 : Saving output of prerun in jobtype1.prerun.txt
<server01> jobtype3 : Saving output of prerun in jobtype3.prerun.txt
<server01> jobtype1 : Saving output of prerun in jobtype1.prerun.txt
...
{
  "fio version" : "fio-3.28",
  "timestamp" : 1637294131,
  "time" : "Thu Nov 18 19:55:31 2021",
  "global options" : {
...

If this makes any difference:

• job definition file defines 3 separate types of jobs (using ioengines filecreate, filestat, filedelete), each with numjobs=4
• job is running in client/server mode with 3 separate servers