Fix portability issues in the new gist pageinspect test.

Fix portability issues in the new gist pageinspect test.

1. The raw bytea representation of the point-type keys used in the test
   depends on endianess. Remove the raw key_data column from the test.

2. The items stored on non-leftmost gist page depends on how many items
   git on the other pages. This showed up as a failure on 32-bit i386
   systems. To fix, only test the gist_page_items() function on the
   leftmost leaf page.

Per Andrey Borodin and the buildfarm.

Discussion: https://www.postgresql.org/message-id/9FCEC1DC-86FB-4A57-88EF-DD13663B36AF%40yandex-team.ru