cleanup(storage): simplify one of the unit tests

[coryan]

The unit test for storage::RetryResumableUploadSession gets shorter
and more readable if one uses the googlemock features for verifying
things happen in a certain sequence.

Part of the work for #7835

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[codecov]

Codecov Report

Merging #7857 (b7e2e67) into main (19b7ec7) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7857      +/-   ##
==========================================
- Coverage   95.12%   95.11%   -0.02%     
==========================================
  Files        1284     1284              
  Lines      116200   115986     -214     
==========================================
- Hits       110539   110322     -217     
- Misses       5661     5664       +3     

Impacted Files	Coverage Δ
google/cloud/storage/client_write_object_test.cc	100.00% <ø> (ø)
...ge/internal/retry_resumable_upload_session_test.cc	99.58% <100.00%> (-0.11%)	⬇️
...storage/internal/retry_resumable_upload_session.cc	99.08% <0.00%> (-0.92%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	85.52% <0.00%> (-0.66%)	⬇️
google/cloud/storage/client.cc	86.58% <0.00%> (-0.64%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	90.10% <0.00%> (+0.35%)	⬆️

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