[TEST ONLY] test fixes for 2.11 server changes

[levb]

• corrected JetStreamMgtStreams to allow for server-added metadata introduced in https://github.com/nats-io/nats-server/pull/5850
• protected JetStreamSubscribeFlowControl against rate limiting introduced in https://github.com/nats-io/nats-server/pull/5796

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.67%. Comparing base (1553d4a) to head (e727531). Report is 15 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #798 +/- ## ========================================== + Coverage 68.71% 70.67% +1.96% ========================================== Files 39 47 +8 Lines 15207 15146 -61 Branches 3143 3096 -47 ========================================== + Hits 10449 10705 +256 + Misses 1700 1455 -245 + Partials 3058 2986 -72 ```

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