[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 15 minutes and 50 seconds (queued for 4 seconds)
Child pipeline (parent) latest detached
Status Name Job ID Coverage
  Configure
passed configure #38735

00:00:52

 
  Linting
passed black (python) #38738

00:00:43

passed pylint-flake8 (python) #38740

00:00:49

 
  Select
passed select tests #38743

00:01:27

 
  Build
passed compile cpp #38747

00:04:46

 
  Test
passed test cpp #38748

00:08:32

passed test python #38749

00:01:15