[Flaky Test] filebeat/input/filestream/internal/input-logfile TestDefaultHarvesterGroup/assert_a_harvester_can_be_stopped_and_removed_from_bookkeeper

## Flaky Test

* **Test Name:** TestDefaultHarvesterGroup/assert_a_harvester_can_be_stopped_and_removed_from_bookkeeper
* **Link:** https://github.com/elastic/beats/blob/master/filebeat/input/filestream/internal/input-logfile/harvester_test.go#L134-L149
* **Branch:** master
* **Artifact Link:** https://beats-ci.elastic.co//blue/rest/organizations/jenkins/pipelines/Beats/pipelines/beats/pipelines/PR-24312/runs/8/steps/11159/log/?start=0

### Stack Trace

```
[2021-03-24T14:20:56.996Z] === FAIL: filebeat/input/filestream/internal/input-logfile TestDefaultHarvesterGroup/assert_a_harvester_can_be_stopped_and_removed_from_bookkeeper (0.00s)
[2021-03-24T14:20:56.996Z] harvester_test.go:108:
[2021-03-24T14:20:56.996Z] Error Trace: harvester_test.go:108
[2021-03-24T14:20:56.996Z] harvester_test.go:151
[2021-03-24T14:20:56.996Z] Error: Should be false
[2021-03-24T14:20:56.996Z] Test: TestDefaultHarvesterGroup/assert_a_harvester_can_be_stopped_and_removed_from_bookkeeper
[2021-03-24T14:20:56.996Z] --- FAIL: TestDefaultHarvesterGroup/assert_a_harvester_can_be_stopped_and_removed_from_bookkeeper (0.00s)
[2021-03-24T14:20:56.997Z]
[2021-03-24T14:20:56.997Z] === FAIL: filebeat/input/filestream/internal/input-logfile TestDefaultHarvesterGroup (0.03s)
[2021-03-24T14:20:56.997Z]
[2021-03-24T14:20:56.997Z] DONE 719 tests, 5 skipped, 2 failures in 51.535s
[2021-03-24T14:20:57.260Z] Error: go test returned a non-zero value: exit status 1
script returned exit code 1
```

Start with filebeat/input/filestream/internal/input-logfile/harvester_test.go, especially the failure helper at lines 108 and 134-151. Run TestDefaultHarvesterGroup and inspect why the harvester is still present when the stop-and-remove assertion runs; done means the targeted test passes reliably without the reported flake.

Written by the indexing model from the issue text.