test(storage): reduce iterations in integration test

[coryan]

WIP

I made this change because another PR #7563 keeps failing, and (1) that PR should not affect this test, (2) the failure is specific to one build, and (3) I am like 😕 and want to rule one some weird thing in Kokoro or something.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7564 (f2dbf5b) into main (6f47d5b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7564   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files        1381     1381           
  Lines      119589   119588    -1     
=======================================
+ Hits       111595   111599    +4     
+ Misses       7994     7989    -5     

Impacted Files	Coverage Δ
...gle/cloud/storage/tests/thread_integration_test.cc	93.44% <100.00%> (-0.06%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.02% <0.00%> (-0.08%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.62% <0.00%> (+0.37%)	⬆️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
google/cloud/bigtable/internal/common_client.cc	97.14% <0.00%> (+1.42%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.51% <0.00%> (+2.19%)	⬆️

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[google-cloud-cpp-bot]

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[coryan]

It times out with just a comment:

3fae374

https://fusion2.corp.google.com/invocations/96d4874d-c905-4393-934f-1fc9c68ce5b8/targets/cloud-cpp%2Fgithub%2Fgoogle-cloud-cpp%2Fmain%2Fmacos%2Fbazel-presubmit;config=default/log

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[devbww]

s/WIP//