test: Allow tests to run on any board

Due to a recent change, tests are limited to running on sandbox only.
Correct this so that any architecture can run them.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Sean Anderson <seanga2@gmail.com>
Fixes: c79705ea ("test: Move dm_test_init() into test-main.c")
Tested-by: Sean Anderson <seanga2@gmail.com>