[azservicebus,azeventhubs] Latest golangci-lint lint warnings should be fixed (they're failing pipelines)

[richardpark-msft]

The latest update for golangci-lint is flagging some more lint issues. I spot checked a few and they look legitimate and simple to fix. Quite a few are in _test.go files, which should make them even simpler.

• azeventhubs: https://dev.azure.com/azure-sdk/internal/_build?pipelineNameFilter=azeventhubs
• azservicebus: https://dev.azure.com/azure-sdk/internal/_build?pipelineNameFilter=azservicebus