page_zip_validate(), page_zip_validate_low(): Add a parameter for the
B-tree index.

page_zip_validate_low(): If the page contents does not match, check
that the record link chains match. Furthermore, if dict_index_t is
passed, check that the records match. (This reduces coverage a bit: if
index=NULL, we will ignore differences in record contents, that is,
the page payload.)

rb:1264 approved by Inaam Rana