* test/ruby/test_regexp.rb
  (TestRegexp#test_options_in_look_behind)
  (TestRegexp#assert_match_at): Add tests for another problem
  fixed in Onigmo 5.13.5.  Previously Onigmo did not allow option
  enclosures in look-behind, which makes it impossible to
  interpolate a regexp into another in the middle of a look-behind
  pattern.  cf. https://github.com/k-takata/Onigmo/pull/17

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41792 b2dd03c8-39d4-4d8f-98ff-823fe69b080e