Add LSP test case

[nitely]

Tests for a (potential) stripped down LSP transport.

Things LSP needs to support:

• Processing requests concurrently (ie: RPC handlers can run concurrently)
• Sending requests to the connected client (ie: bidi communication)
• Cancelling a given in progress RPC request by its request id (ie: RPC handlers can be cancelled)
• stdio transport (not implemented)

[nitely]

mmh, but this does not add much that is not already tested. I was thinking if some of it could be implemented in json-rpc itself (ie: concurrent rpc handlers), the improvements could be tested with this instead of the LSP... but for now it does not seem worth to add it to the test suite.