Fix LT-15512 issues with unnecessary lift import residue

* Add or tweak tests to show that data imported into custom fields
  does not end up placed in the LiftResidue
* minor refactoring of test code to improve usability/readability
* remove code that unnecessarily added LiftResidue for every trait in
  an example sentence.

Change-Id: Ib28a07c89698c667d3ddbb8f1369309bab024401