[test][freeflow] Fix header of error csv file.

Previously it was (incorrectly) assumed that pressure is index 0 in primary variables,
u is index 1 and v is index 2.
7 jobs for !2829 with fix/ffErrorCsvFile in 14 minutes and 45 seconds (queued for 4 seconds)
Child pipeline (parent) latest detached
Status Name Job ID Coverage
  Configure
passed configure #38722

00:02:48

 
  Linting
passed black (python) #38723

00:01:26

passed pylint-flake8 (python) #38724

00:01:13

 
  Select
passed select tests #38725

00:02:20

 
  Build
passed compile cpp #38728

00:02:12

 
  Test
passed test cpp #38730

00:07:06

passed test python #38734

00:00:43