package/rt-tests: fix _SITE download url

Upstream moved old versions in an "older" directory. The current version
in Buildroot was moved to this directory and fail to download.

This commit simply updates the _SITE url to reflect this change.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/8514189435

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/rt-tests/rt-tests.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-RT_TESTS_SITE = $(BR2_KERNEL_MIRROR)/linux/utils/rt-tests
+RT_TESTS_SITE = $(BR2_KERNEL_MIRROR)/linux/utils/rt-tests/older
 RT_TESTS_SOURCE = rt-tests-$(RT_TESTS_VERSION).tar.xz
 RT_TESTS_VERSION = 2.7
 RT_TESTS_LICENSE = GPL-2.0+