Currently the test uses a sequence of 1024-bytes buffers. This lets
minio server actively de-duplicate those blocks by page boundary (it's a
guess, but it it's truish because minio reports back equivalent ETags
for lots of uploading parts). Make the buffer not be power of two so
that when squashed together the resulting 2^X buffers don't get equal.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>