tests: IO: Constify write buffer pointes as well.
tests: IO: Constify write buffer pointes as well.

Fixes warnings introduced by:

commit a53b47eab687c422a5a0b887ee9616c62ecd7723
Author: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>
Date:   Sat Feb 10 22:14:30 2018 +0100

    loaders: IO: Constify buf pointer for write()

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

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