tests/9p: simplify callers of treaddir()

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

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <7cec6f2c7011a481806c34908893b7282702a7a6.1664917004.git.qemu_oss@crudebyte.com>