Since 13c0c17c (bigfile/zodb: Format #1 which is optimized for small
changes) each ZBlk is split into chunks, and the procedure to load/save
blk data collects/disassembles the chunks.

Previously in bigfile/zodb tests, we were testing by verifying only [0]
element in each block, so this way most code for chunks
collection/disassembling was not tested.

Fix this aspect by making sure whole block content is as would-be
expected in tests.