test: spl: Fix spl_test_load not failing if fname doesn't exist

test: spl: Fix spl_test_load not failing if fname doesn't exist

Returning a negative value from a unit test doesn't automatically fail the
test.  We have to fail an assertion. Modify the test to do so.

This now causes the test to count as a failure on VPL. This is because the
fname of SPL (and U-Boot) is generated with make_exec in os_jump_to_image.
The original name of SPL is gone, and we can't determine the name of U-Boot
from the generated name.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>