[8.x](backport #41230) Re-enable TestGroup_Go and fix flaky behavior

[mergify]

Proposed commit message

This PR re-enables the TestGroup_Go test, which was skipped in #41223. It also includes fixes to mitigate flakiness in the test logger caused by timming issues and ensures proper waiting for success conditions within the tests.

How to test this PR locally

$ cd filebeat/input/filestream/internal/task 
$ go test -c
$ go run golang.org/x/tools/cmd/stress@latest ./task.test -test.run TestGroup_Go -test.v

Related issues

• Fixes TestGroup_Go - not all goroutines finished/"task.Group: semaphore acquire failed, was the task group closed? err: context canceled #41218

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  This is an automatic backport of pull request Re-enable TestGroup_Go and fix flaky behavior #41230 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)