[8.x](backport #42097) test: sync access to countRun counter in TestRunnerFactory

[mergify]

Proposed commit message

countRun is incremented and checked in different goroutines

to avoid race conditions, only mark the workgroup as done after incrementing the counter

==================
WARNING: DATA RACE
Read at 0x00c000358fc8 by goroutine 10:
  github.com/elastic/beats/v7/filebeat/input/v2/compat.TestRunnerFactory_CheckConfig.func2()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat_test.go:127 +0xa31
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      testing/testing.go:1743 +0x44

Previous write at 0x00c000358fc8 by goroutine 14:
  github.com/elastic/beats/v7/filebeat/input/v2/compat.TestRunnerFactory_CheckConfig.func2.1.2()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat_test.go:89 +0x4f
  github.com/elastic/beats/v7/filebeat/input/v2/internal/inputest.(*MockInput).Run()
      github.com/elastic/beats/v7/filebeat/input/v2/internal/inputest/inputest.go:80 +0x124
  github.com/elastic/beats/v7/filebeat/input/v2/compat.(*runner).Start.func1()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat.go:136 +0x3c7

Goroutine 10 (running) created at:
  testing.(*T).Run()
      testing/testing.go:1743 +0x825
  github.com/elastic/beats/v7/filebeat/input/v2/compat.TestRunnerFactory_CheckConfig()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat_test.go:67 +0x56
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      testing/testing.go:1743 +0x44

Goroutine 14 (finished) created at:
  github.com/elastic/beats/v7/filebeat/input/v2/compat.(*runner).Start()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat.go:133 +0x16f
  github.com/elastic/beats/v7/filebeat/input/v2/compat.TestRunnerFactory_CheckConfig.func2()
      github.com/elastic/beats/v7/filebeat/input/v2/compat/compat_test.go:116 +0x7a9
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      testing/testing.go:1743 +0x44
==================
--- FAIL: TestRunnerFactory_CheckConfig (0.00s)
    --- FAIL: TestRunnerFactory_CheckConfig/does_not_cause_input_ID_duplication (0.00s)
        testing.go:1399: race detected during execution of test

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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---

This is an automatic backport of pull request #42097 done by [Mergify](https://mergify.com).

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