add some tests for rfind

Dependencies

• [2] LXTTOB33 extract a couple of files

Change contents

• edit in search.lua at line 144
  [2.52064]
  +

  + function test_rfind()

  + check_eq(rfind('abc', 'c'), 3, 'final char')

  + check_eq(rfind('acbc', 'c', 3), 2, 'previous char')

  + check_nil(rfind('abc', 'd'), 'missing char')

  + check_nil(rfind('abc', 'c', 2), 'no more char')

  + end