test(eviction agent): attempt to stabilize flaky test

```
%%% emqx_eviction_agent_SUITE ==> t_session_serialization: FAILED
%%% emqx_eviction_agent_SUITE ==>
Failure/Error: ?assertEqual(1, emqx_eviction_agent : session_count ( ))
  expected: 1
       got: 2
      line: 348
```

apps/emqx_eviction_agent/test/emqx_eviction_agent_SUITE.erl

@@ -343,9 +343,13 @@ t_session_serialization(_Config) ->
 
     ok = emqx_eviction_agent:disable(test_eviction),
 
-    ?assertEqual(
-        1,
-        emqx_eviction_agent:session_count()
+    ?retry(
+        200,
+        10,
+        ?assertEqual(
+            1,
+            emqx_eviction_agent:session_count()
+        )
     ),
 
     ?assertMatch(