Review/Update Diagnostic Tests #5329
Description
To avoid issues with updates to Diagnostic and Diagnostic<TState, TStateEnd>, we should ensure our tests replicate the APM agent behaviour - Store current Activity on start and look for it on stop e.g. in a concurrent dictionary.
Relates to #5326
Existing tests: https://github.com/elastic/apm-agent-dotnet/blob/master/test/Elastic.Apm.Elasticsearch.Tests/VirtualElasticsearchTests.cs