test: Fix SPL tests not being run

SPL doesn't have OF_LIVE enabled, so we can only run tests with a flat
tree. Don't skip them even if they don't use the devicetree.

Fixes: 6ec5178c ("test: Skip flat-tree tests if devicetree is not used")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>