Fixed incorrect ("overoptimized") implementation of #addEssentialFields() in PTS

  for nested structures there were no way to start tracking for any inner structure field if the
  structure always occurred as inner (see comment in the method).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14464 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c