test: dm: spi: Fix sandbox dm_test_spi_find()

Per sandbox_cs_info(), sandbox spi controller only supports chip
select 0. Current test case tries to locate devices on chip select
1, and any call to spi_get_bus_and_cs() or spi_cs_info() with cs
number 1 should not return 0.

This updates the test case to handle it correctly.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>