This patch improves the test which compares the performance of the
different ZBlk formats by using test data which better represents the
behaviour of a real-world instance that actually uses 'wendelin.core'
with the 'many-small-appends' use-case.

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Running 'bigfile/tests/test-zblk-fmt' with improved test data results
in:

Run append tests
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Set change_percentage_set to 0.014
Set change_count to 500
Set arrsize to 500000
Set change_type to append

Run tests with format auto:

	ZODB storage size: 29.428379 MB
	Access time: 0.785 ms / blk  (initially cold; might get warmer during benchmark)

Run tests with format ZBlk0:

	ZODB storage size: 538.109848 MB
	Access time: 0.754 ms / blk  (initially cold; might get warmer during benchmark)

Run tests with format ZBlk1:

	ZODB storage size: 16.844756 MB
	Access time: 88.665 ms / blk  (initially cold; might get warmer during benchmark)

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