Cleanup managed channel instances in jaeger remote sampler tests

[jack-berg]

Removes build output like:

[Test worker] ERROR io.grpc.internal.ManagedChannelOrphanWrapper - *~*~*~ Previous channel ManagedChannelImpl{logId=5, target=127.0.0.1:57230} was garbage collected without
  being shut down! ~*~*~*
          Make sure to call shutdown()/shutdownNow()
      java.lang.RuntimeException: ManagedChannel allocation site

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (839f235) to head (6dd0fd0).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8183   +/-   ##
=========================================
  Coverage     90.29%   90.30%           
- Complexity     7650     7651    +1     
=========================================
  Files           843      843           
  Lines         23059    23061    +2     
  Branches       2309     2310    +1     
=========================================
+ Hits          20822    20826    +4     
+ Misses         1519     1516    -3     
- Partials        718      719    +1     

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