minor: Avoid flake in rabbit_mgmt_http_health_checks_SUITE

The `below_node_connection_limit_test` and `ready_to_serve_clients_test`
cases could possibly flake because `is_quorum_critical_single_node_test`
uses the channel manager in `rabbit_ct_client_helpers` to open a
connection. This can cause the line

     true = lists:all(fun(E) -> is_pid(E) end, Connections),

to fail to match. The last connection could have been rejected if the
channel manager kept its connection open, so instead of being a pid the
element would have been `{error, not_allowed}`.

With `rabbit_ct_client_helpers:close_channels_and_connection/2` we can
reset the connection manager and force it to close its connection.

Author: the-mikedavis

deps/rabbitmq_management/test/rabbit_mgmt_http_health_checks_SUITE.erl

@@ -478,6 +478,7 @@ below_node_connection_limit_test(Config) ->
     ?assertEqual(<<"ok">>, maps:get(status, Check0)),
 
     %% Set the connection limit low and open 'limit' connections.
+    rabbit_ct_client_helpers:close_channels_and_connection(Config, 0),
     Limit = 10,
     rabbit_ct_broker_helpers:rpc(
       Config, 0, application, set_env, [rabbit, connection_max, Limit]),
@@ -506,6 +507,7 @@ ready_to_serve_clients_test(Config) ->
     true = rabbit_ct_broker_helpers:unmark_as_being_drained(Config, 0),
 
     %% Set the connection limit low and open 'limit' connections.
+    rabbit_ct_client_helpers:close_channels_and_connection(Config, 0),
     Limit = 10,
     rabbit_ct_broker_helpers:rpc(
       Config, 0, application, set_env, [rabbit, connection_max, Limit]),