tests/9p: simplify callers of twrite()

Now as twrite() is using a declarative approach, simplify the
code of callers of this function.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <7f280ec6a1f9d8afed46567a796562c4dc28afa9.1664917004.git.qemu_oss@crudebyte.com>