cmd: testing: add option for block_dev_test to start at given block number
cmd: testing: add option for block_dev_test to start at given block number

This patch adds functionality for 'block_dev_test' command to
specify the block number at which the command should start. It also
adds information about this option to be displayed in the help
message. This fixes part of #1709.

Signed-off-by: Mohana Datta Yelugoti <cdbecbb57185356557746f287212272f4e70d029@gmail.com>

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