cleanup!: restrict testing_util build visibility

[devjgm]

Part of #3701

Change the default build visibility to private. Expose the previous targets as public but deprecated.

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This change is 

[google-cloud-cpp-bot]

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For commit: df5bf5fe309694efc45bcb22aacc5727886aaa12

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8584 (684ba69) into main (c7ffdec) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8584   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files        1462     1462           
  Lines      126954   126954           
=======================================
+ Hits       119210   119216    +6     
+ Misses       7744     7738    -6     

Impacted Files	Coverage Δ
google/cloud/bigtable/internal/common_client.h	94.02% <0.00%> (-5.98%)	⬇️
...le/cloud/spanner/database_admin_connection_test.cc	99.70% <0.00%> (+0.29%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.75% <0.00%> (+0.49%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️
google/cloud/spanner/database_admin_connection.cc	85.71% <0.00%> (+0.57%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	86.18% <0.00%> (+0.65%)	⬆️
google/cloud/bigtable/internal/common_client.cc	97.14% <0.00%> (+1.42%)	⬆️

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[coryan]

I would have expected these needed the *_internal + alias treatment too?

[devjgm]

I would have expected these needed the *_internal + alias treatment too?

I can add that later today. I didn't bother originally because https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/testing_util/README.md says

The functions and types provided here are not intended for general use, and are subject to change without notice.

[coryan]

I had forgotten that from the README, SGTM.

[devjgm]

I had forgotten that from the README, SGTM.

Sorry, too late :-) Exposed publicly and deprecated. We can proceed either way with minimal effort. Do you have a preference how you'd like to proceed?

[google-cloud-cpp-bot]

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For commit: 684ba6934193de0cf388830689a6178a0f235fdd

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