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

[pichlermarc]

Which problem is this PR solving?

All the functionality that's duplicate-tested in all the gRPC exporters is now tested in the exporter base packages as well as in @opentelemetry/otlp-transformer for serialization.

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

• gRPC is indeed used
• using an address connects to the correct address
• the export succeeds and nothing additional is sent

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. The tests had an additional check that ensured that the export also works via Unix domain sockets, so I added another test in the @opentelemtry/otlp-grpc-exporter base package to avoid loosing coverage there.

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 94.59%. Comparing base (e35f3d6) to head (c5304c6).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Misses        432      432