cleanup!: change bazel visibility to private for {tests,quickstarts,examples,samples,benchmarks}

[devjgm]

Part of #3701

These changes are only affecting whole BUILD files that are in a directory that is obviously not intended to be part of a user-facing API. These are (integration) tests, examples, samples, benchmarks, and quickstarts.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: b83ff7f6b275b5078a729aa937706498da2ec04e

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #8578 (b83ff7f) into main (8195b41) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #8578      +/-   ##
==========================================
- Coverage   93.94%   93.94%   -0.01%     
==========================================
  Files        1462     1462              
  Lines      126883   126883              
==========================================
- Hits       119203   119202       -1     
- Misses       7680     7681       +1     

Impacted Files	Coverage Δ
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.50%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️

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