Test Coverage: Deadlines

[csvance]

Add some test coverage for deadlines:

1. Whether they work
2. That deadlines are properly converted to gRPC header values (grpc-timeout)

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.13%. Comparing base (4e9aeea) to head (d705773).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   76.36%   79.13%   +2.76%     
==========================================
  Files           7        7              
  Lines         512      508       -4     
==========================================
+ Hits          391      402      +11     
+ Misses        121      106      -15     

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