test: fix submission watcher test

[j4m1ef0rd]

Pull Request

Closes: PRO-1718

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have written sufficient tests.
• I have written and tested required migrations.
• I have updated documentation where appropriate.

Summary

Changed the jsonrpsee::Subscription into a Stream so we can return an empty stream in the test.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 71%. Comparing base (5baa3dd) to head (2986d80).

Files with missing lines	Patch %	Lines
...ne/src/state_chain_observer/client/base_rpc_api.rs	0%	4 Missing ⚠️
...t/extrinsic_api/signed/submission_watcher/tests.rs	96%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #5358    +/-   ##
======================================
  Coverage     71%     71%            
======================================
  Files        496     497     +1     
  Lines      86485   86447    -38     
  Branches   86485   86447    -38     
======================================
+ Hits       61751   61802    +51     
+ Misses     21971   21864   -107     
- Partials    2763    2781    +18     

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