[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.
5 jobs for !2829 with fix/ffErrorCsvFile
latest detached
Status Name Job ID Coverage
passed full-dune-2.7-clang

passed full-dune-2.7-gcc

passed full-dune-master-gcc

passed minimal-dune-2.7-gcc

  Downstream Modules
passed trigger lecture