Delete duplicated OTLP Exporter tests, move them to the mixin unit test. Fix broken shutdown unit test.#4504

aabmass merged 16 commits intoopen-telemetry:mainopen-telemetry/opentelemetry-python:mainfrom

DylanRussell:fix_broken_testDylanRussell/opentelemetry-python:fix_broken_testCopy head branch name to clipboard