test: spl: Split tests up and use some configs

test: spl: Split tests up and use some configs

In order to make adding new spl unit tests easier, especially when they may
have many dependencies, add some Kconfigs for the existing image test.
Split it into the parts which are generic (such as callbacks) and the
test-specific parts.

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