test_io.rb: test with holes

* test/ruby/test_io.rb (test_seek, test_seek_symwhence): add tests
  of SEEK_DATA and SEEK_HOLE with holes.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45659 b2dd03c8-39d4-4d8f-98ff-823fe69b080e