test: boot: fix bootdev_test_any for when DSA_SANDBOX is disabled

test: boot: fix bootdev_test_any for when DSA_SANDBOX is disabled

When DSA_SANDBOX is not set, the sandbox tests fail as follows:

 $ ./test/py/test.py --build-dir=$(pwd) -k bootdev_test_any
 [...]
 Test: bootdev_test_any: bootdev.c
 test/boot/bootdev.c:156, bootdev_test_any(): "mmc2" = media->name: Expected "mmc2", got "mmc0"
 [...]

This is due to the device list containing two less entries than
expected. Therefore, adjust the expected index to be two less when
DSA_SANDBOX is disabled.

The actual use case is NET_LWIP=y (to be introduced in later patches)
which implies DSA_SANDBOX=n for the time being.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>