[tests]: fix buffer deadlock bug (#218)

* [tests]: buffer deadlock

Fix a bad merge changed to `request_channel_capacity` to `subscription_channel_capacity`

https://github.com/paritytech/jsonrpsee/commit/7dc9435e225bcc1887a2532e830266d7b1ecd0af#diff-aeefe6b3d3f8b1b5248d6bb4c2888cf2d6802dbf82515f7b02b6a89a1400228fL188

* Update tests/src/lib.rs

* fix build: `max_concurrent_requests_capacity`