get_all_tests

  Description: Returns a list of testfiles in the directories specified by
               the @tests argument.  The relative path is given as well as
               with the testnames returned.  Only files ending with .t are
               returned.  Subdirectories are recursively entered and the test
               files returned within them are returned as well.
  Returntype : listref of strings.
  Exceptions : none
  Caller     : general