test: Tidy up sandbox handling in test-main

This is pretty messy at present since it relies on a SPL_SANDBOX option
that does not exist. Use the normal options instead, so that it will work
with split config.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/test-main.c b/test/test-main.c
index ea959f4..b3c30d9 100644
--- a/test/test-main.c
+++ b/test/test-main.c
@@ -46,14 +46,14 @@
  */
 static enum fdtchk_t fdt_action(void)
 {
-	/* Do a copy for sandbox (but only the U-Boot build, not SPL) */
-	if (CONFIG_IS_ENABLED(SANDBOX))
-		return FDTCHK_COPY;
-
 	/* For sandbox SPL builds, do nothing */
-	if (IS_ENABLED(CONFIG_SANDBOX))
+	if (IS_ENABLED(CONFIG_SANDBOX) && IS_ENABLED(CONFIG_SPL_BUILD))
 		return FDTCHK_NONE;
 
+	/* Do a copy for sandbox (but only the U-Boot build, not SPL) */
+	if (IS_ENABLED(CONFIG_SANDBOX))
+		return FDTCHK_COPY;
+
 	/* For all other boards, do a checksum */
 	return FDTCHK_CHECKSUM;
 }