Codify stance against using java-test-fixtures plugin

[jack-berg]

See https://github.com/open-telemetry/opentelemetry-java/pull/6695#discussion_r1750447194:

In hindsight, adding java-test-fixtures was overly hasty, since as shown in https://github.com/open-telemetry/opentelemetry-java/issues/6693 it impacts the pom.xml and publishes additional artifacts we didn't intend / account for. When we need to share test code in this repository, we simply add a *:testing-internal module and omit the otel.publish-conventions. I suspect this pattern was selected in the past after investigating test fixtures and seeing these types of side affects.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.09%. Comparing base (b0643fe) to head (a35e63b). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6696 +/- ## ========================================= Coverage 90.09% 90.09% Complexity 6390 6390 ========================================= Files 711 711 Lines 19333 19333 Branches 1891 1891 ========================================= Hits 17418 17418 Misses 1335 1335 Partials 580 580 ```

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