Fix LT-15516: Blank reversals multiply on import

* Do not create reversal entries in FLEx for blank
  reversal entries found in the LIFT file.
* Minor refactoring of the preceding test
* Test empty reversals are eleminated without affecting
  non empty ones.
* Change test utilities to use slightly randomized file names to
  reduce chance of file collision with previous test failures
  or in cases where tests might run in parallel.

Change-Id: I3e2aa4b98ce2b728622611fdeee3d6bd6276d1b1