[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 8 minutes and 8 seconds (queued for 4 seconds)
Child pipeline (parent) latest detached
Status Name Job ID Coverage
  Configure
passed configure #38727

00:00:52

 
  Linting
passed black (python) #38731

00:00:44

passed pylint-flake8 (python) #38733

00:00:49

 
  Select
passed select tests #38737

00:01:44

 
  Build
passed compile cpp #38741

00:02:55

 
  Test
passed test cpp #38744

00:02:35

passed test python #38746

00:01:23