Attempt to de-flake

This commit attempts to remove the following flake:
```
{amqp_client_SUITE,server_closes_link,1113}
{badmatch,[<14696.3530.0>,<14696.3453.0>]}
```
by waiting after each test case until sessions were de-registered from
the 1st RabbitMQ node.

Author: ansd

deps/rabbit/test/amqp_client_SUITE.erl

@@ -258,6 +258,8 @@ init_per_testcase(Testcase, Config) ->
 end_per_testcase(Testcase, Config) ->
     %% Assert that every testcase cleaned up.
     eventually(?_assertEqual([], rpc(Config, rabbit_amqqueue, list, []))),
+    %% Wait for sessions to terminate before starting the next test case.
+    eventually(?_assertEqual([], rpc(Config, rabbit_amqp_session, list_local, []))),
     %% Assert that global counters count correctly.
     eventually(?_assertMatch(#{publishers := 0,
                                consumers := 0},