chore(exporter-otlp-*-http): clean up tests

[pichlermarc]

Which problem is this PR solving?

All the functionality that's duplicate-tested in all the HTTP/json exporters is now tested in the exporter base packages as well as in @opentelemetry/otlp-transformer for serialization. This PR is similar to #5196, but for the HTTP exporters.

This PR removes the tests from the individual exporters and replaces them with a simple check that tests if

• http/xhr/sendBeacon is indeed used
• the export succeeds

The previous implementation was heavily relying on constructing export data and asserting the data that is sent, which makes changes to the SDK's internal structure very difficult. This testing is now all handled in @opentelemetry/otlp-transformer so there should be no need to test this again in every individual exporter.

I also added a comment for future contributors that this tests file is not where they should add any tests that concern base components.

Partially enables #4583 (types exported from @opentelemetry/otlp-transformer are used in testing, by removing test usages, we can remove types that were only used in tests from the public API)

Type of change

• Internal test update

How Has This Been Tested?

• Changed tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.58%. Comparing base (556eecf) to head (eebcf22).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   94.58%   94.58%           
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  Files         314      314           
  Lines        7993     7993           
  Branches     1611     1611           
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  Hits         7560     7560           
  Misses        433      433