transport: Set buffer pool in tests

[arjan-bal]

This PR correctly sets the buffer pool for test clients and servers not created through the public gRPC API. This allows non-test code to assume the buffer pool is always present.

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.24%. Comparing base (363018c) to head (3291713).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8688      +/-   ##
==========================================
- Coverage   83.43%   83.24%   -0.20%     
==========================================
  Files         415      415              
  Lines       32195    32197       +2     
==========================================
- Hits        26863    26803      -60     
- Misses       3980     4007      +27     
- Partials     1352     1387      +35     

Files with missing lines	Coverage Δ
internal/transport/http_util.go	85.66% <ø> (-8.42%)	⬇️

... and 17 files with indirect coverage changes

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