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/str_ut.c b/test/str_ut.c
index cd50455..359d7d4 100644
--- a/test/str_ut.c
+++ b/test/str_ut.c
@@ -107,9 +107,8 @@
 
 int do_ut_str(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 {
-	struct unit_test *tests = ll_entry_start(struct unit_test,
-						 str_test);
-	const int n_ents = ll_entry_count(struct unit_test, str_test);
+	struct unit_test *tests = UNIT_TEST_SUITE_START(str_test);
+	const int n_ents = UNIT_TEST_SUITE_COUNT(str_test);
 
 	return cmd_ut_category("str", "str_", tests, n_ents, argc, argv);
 }