test: Add a macros for finding tests in linker_lists

At present we use the linker list directly. This is not very friendly, so
add a helpful macro instead. This will also allow us to change the naming
later without updating this code.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-dm.c b/test/dm/test-dm.c
index cb4f995..729becf 100644
--- a/test/dm/test-dm.c
+++ b/test/dm/test-dm.c
@@ -22,8 +22,8 @@
 
 int dm_test_run(const char *test_name)
 {
-	struct unit_test *tests = ll_entry_start(struct unit_test, dm_test);
-	const int n_ents = ll_entry_count(struct unit_test, dm_test);
+	struct unit_test *tests = UNIT_TEST_SUITE_START(dm_test);
+	const int n_ents = UNIT_TEST_SUITE_COUNT(dm_test);
 	int ret;
 
 	ret = ut_run_list("driver model", "dm_test_", tests, n_ents, test_name);