test: flaky TestRunManager_Reloader_newRunnerError

[carsonip]

TestRunManager_Reloader_newRunnerError is flaky e.g. https://github.com/elastic/apm-server/actions/runs/19664829682/job/56319090910?pr=19687

WARNING: DATA RACE
Write at 0x00c0001bc20b by goroutine 151:
  testing.tRunner.func1()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1921 +0x904
  runtime.deferreturn()
      /opt/hostedtoolcache/go/1.25.4/x64/src/runtime/panic.go:589 +0x5d
  testing.(*T).Run.gowrap1()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1997 +0x44

Previous read at 0x00c0001bc20b by goroutine 159:
  testing.(*outputWriter).writeLine()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1159 +0x70
  testing.(*outputWriter).Write()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1149 +0x2ad
  testing.(*common).log()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1040 +0x164
  testing.(*common).Logf()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1191 +0x8e
  go.uber.org/zap/zaptest.TestingWriter.Write()
      /home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zaptest/logger.go:146 +0x119
  go.uber.org/zap/zaptest.(*TestingWriter).Write()
      <autogenerated>:1 +0x74
  go.uber.org/zap/zapcore.(*ioCore).Write()
      /home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zapcore/core.go:99 +0x18d
  go.uber.org/zap/zapcore.(*CheckedEntry).Write()
      /home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zapcore/entry.go:253 +0x1ec
  go.uber.org/zap.(*SugaredLogger).log()
      /home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/sugar.go:355 +0x12c
  go.uber.org/zap.(*SugaredLogger).Errorf()
      /home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/sugar.go:216 +0x239
  github.com/elastic/elastic-agent-libs/logp.(*Logger).Errorf()
      /home/runner/go/pkg/mod/github.com/elastic/elastic-agent-libs@v0.26.0/logp/logger.go:251 +0x1ec
  github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).watchErrChan()
      /home/runner/go/pkg/mod/github.com/elastic/beats/v7@v7.0.0-alpha2.0.20251105085148-a4226fe882c0/x-pack/libbeat/management/managerV2.go:472 +0x16d
  github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).Start.gowrap1()
      /home/runner/go/pkg/mod/github.com/elastic/beats/v7@v7.0.0-alpha2.0.20251105085148-a4226fe882c0/x-pack/libbeat/management/managerV2.go:292 +0x4f

Goroutine 151 (running) created at:
  testing.(*T).Run()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1997 +0x9d2
  testing.runTests.func1()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:2477 +0x85
  testing.tRunner()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1934 +0x21c
  testing.runTests()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:2475 +0x96c
  testing.(*M).Run()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:2337 +0xed4
  main.main()
      _testmain.go:73 +0x164

Goroutine 159 (running) created at:
  github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).Start()
      /home/runner/go/pkg/mod/github.com/elastic/beats/v7@v7.0.0-alpha2.0.20251105085148-a4226fe882c0/x-pack/libbeat/management/managerV2.go:292 +0x259
  github.com/elastic/apm-server/internal/beatcmd.TestRunManager_Reloader_newRunnerError()
      /home/runner/work/apm-server/apm-server/internal/beatcmd/beat_test.go:628 +0x161a
  testing.tRunner()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1934 +0x21c
  testing.(*T).Run.gowrap1()
      /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1997 +0x44