Add test-case for explicit file conflict variants

Add test-case for explicit file conflict variants

- Three-way tests for explicit (aka manually added) file conflicts:
  conflict within added set, and conflict in transaction <-> database.
- The problem message rpm creates for the third case is pretty bad,
  it misses the fact its on an installed package and gets the name
  wrong too. Better than not detecting though, and the message has
  been broken for a long time so not related to the recent other
  changes.
(cherry picked from commit 909aab0916ef851ea78df4dbd7bd8c9d2045c7ab)