sandbox: test: Tidy up spl_test_load() calls

Use the new sandbox_find_next_phase() function, which does what is needed
here.
Signed-off-by: NSimon Glass <sjg@chromium.org>

test/image/spl_load.c

@@ -56,7 +56,6 @@ struct image_header *spl_get_load_buffer(ssize_t offset, size_t size)
 
 static int spl_test_load(struct unit_test_state *uts)
 {
-	const char *cur_prefix, *next_prefix;
 	struct spl_image_info image;
 	struct image_header *header;
 	struct text_ctx text_ctx;
@@ -69,10 +68,7 @@ static int spl_test_load(struct unit_test_state *uts)
 	load.bl_len = 512;
 	load.read = read_fit_image;
 
-	cur_prefix = spl_phase_prefix(spl_phase());
-	next_prefix = spl_phase_prefix(spl_next_phase());
-	ret = os_find_u_boot(fname, sizeof(fname), true, cur_prefix,
-			     next_prefix);
+	ret = sandbox_find_next_phase(fname, sizeof(fname), true);
 	if (ret) {
 		printf("(%s not found, error %d)\n", fname, ret);
 		return ret;