Increase check in live_snapshot_chain

This test allows to check all snapshots, but currently only checks the
main image and last snapshot. Let's check one of the middle snapshots as
well to see it behaves correctly.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/live_snapshot_chain.cfg

@@ -38,6 +38,8 @@
         - long_chain:
             need_reboot = yes
             file_create = yes
+            # Check base, middle and last (set in code) images
             check_base_image_image1 = yes
+            check_base_image_sn32 = yes
             snapshot_chain = "image1"
             snapshot_num = 64