Fix LT-15181 (crash in FixFwData)

* If two custom fields with the same name are on different types
  e.g. LexEntry and LexSense it should not crash when testing the
  file after a merge.
* Modified some existing test data to prove the fix.

Change-Id: I9e02ae27a96ac32b8ed6b7738ba1633e151e1483