[202012] Skip bgp speaker test on storage backend topology (#6921)

Signed-off-by: Neetha John <nejo@microsoft.com>

bgpmon and slb peers were removed for storage backend topology as part of sonic-net/sonic-buildimage#12262 which was causing bgp speaker test to fail on t0-backend topology. Hence skipping this test on backend topo

tests/common/plugins/conditional_mark/tests_mark_conditions.yaml

@@ -32,9 +32,9 @@ bfd/test_bfd.py:
 #######################################
 bgp/test_bgp_speaker.py:
   skip:
-    reason: "M0 topo does not support test_bgp_speaker"
+    reason: "M0/backend topo does not support test_bgp_speaker"
     conditions:
-      - "topo_name in ['m0']"
+      - "topo_name in ['m0', 't0-backend']"
 
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