Flaky test: TestSpawnFailure

Job #1326582878 failed for d1515cf9:

=== RUN   TestSpawnFailure
==================
WARNING: DATA RACE
Write at 0x0000013775f8 by goroutine 44:
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.TestSpawnFailure.func1()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor_test.go:101 +0x3a
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.TestSpawnFailure()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor_test.go:129 +0x789
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1194 +0x202

Previous read at 0x0000013775f8 by goroutine 43:
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.watch()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor.go:129 +0xcde

Goroutine 44 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:1239 +0x5d7
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:1512 +0xa6
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1194 +0x202
  testing.runTests()
      /usr/local/go/src/testing/testing.go:1510 +0x612
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:1418 +0x3b3
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.testMain()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor_test.go:61 +0x859
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.TestMain()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor_test.go:27 +0x38
  main.main()
      _testmain.go:49 +0x271

Goroutine 43 (running) created at:
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.New()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor.go:82 +0x39b
  gitlab.com/gitlab-org/gitaly/v14/internal/supervisor.TestSpawnFailure()
      /builds/gitlab-org/gitaly/internal/supervisor/supervisor_test.go:110 +0x2f8
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1194 +0x202
==================
    testing.go:1093: race detected during execution of test
--- FAIL: TestSpawnFailure (3.05s)
=== CONT  
    testing.go:1093: race detected during execution of test
FAIL
FAIL	gitlab.com/gitlab-org/gitaly/v14/internal/supervisor	5.980s

/cc @zj-gitlab