Flaky test: TestArtifactFetcher_SnapshotOnly

[faec]

TestArtifactFetcher_SnapshotOnly is flaky because of a data race in the test code. Error message:

WARNING: DATA RACE
--
  | Read at 0x00c00037e218 by goroutine 11:
  | github.com/elastic/elastic-agent/pkg/testing.(*writeProgress).printProgress()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:285 +0x239
  | github.com/elastic/elastic-agent/pkg/testing.newWriteProgress.func1()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:267 +0x58
  |  
  | Previous write at 0x00c00037e218 by goroutine 9:
  | github.com/elastic/elastic-agent/pkg/testing.(*writeProgress).Write()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:273 +0xa8
  | io.(*teeReader).Read()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/io/io.go:615 +0xbe
  | io.copyBuffer()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/io/io.go:427 +0x28d
  | io.Copy()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/io/io.go:386 +0x88
  | os.genericReadFrom()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/os/file.go:161 +0x34
  | os.(*File).ReadFrom()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/os/file.go:155 +0x99
  | io.copyBuffer()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/io/io.go:413 +0x1c5
  | io.Copy()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/io/io.go:386 +0x62d
  | github.com/elastic/elastic-agent/pkg/testing.DownloadPackage()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:243 +0x567
  | github.com/elastic/elastic-agent/pkg/testing.(*artifactResult).Fetch()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:110 +0x456
  | github.com/elastic/elastic-agent/pkg/testing.TestArtifactFetcher_SnapshotOnly()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact_test.go:95 +0xdc7
  | testing.tRunner()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1576 +0x216
  | testing.(*T).Run.func1()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1629 +0x47
  |  
  | Goroutine 11 (running) created at:
  | github.com/elastic/elastic-agent/pkg/testing.newWriteProgress()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:267 +0x1dc
  | github.com/elastic/elastic-agent/pkg/testing.DownloadPackage()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:237 +0x484
  | github.com/elastic/elastic-agent/pkg/testing.(*artifactResult).Fetch()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact.go:110 +0x456
  | github.com/elastic/elastic-agent/pkg/testing.TestArtifactFetcher_SnapshotOnly()
  | /Users/admin/builds/bk-agent-prod-orka-1698924985688909064/elastic/elastic-agent/pkg/testing/fetcher_artifact_test.go:95 +0xdc7
  | testing.tRunner()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1576 +0x216
  | testing.(*T).Run.func1()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1629 +0x47
  |  
  | Goroutine 9 (running) created at:
  | testing.(*T).Run()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1629 +0x805
  | testing.runTests.func1()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:2036 +0x8d
  | testing.tRunner()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1576 +0x216
  | testing.runTests()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:2034 +0x87c
  | testing.(*M).Run()
  | /Users/admin/.gvm/versions/go1.20.10.darwin.amd64/src/testing/testing.go:1906 +0xb44
  | main.main()
  | _testmain.go:90 +0x2fc
  | ==================
  | fetcher_artifact.go:285: Downloading artifact progress 100.00%
  | fetcher_artifact.go:252: Completed downloading artifact from https://snapshots.elastic.co/8.8.0-ba3f07b2/downloads/beats/elastic-agent/elastic-agent-8.6.0-SNAPSHOT-darwin-x86_64.tar.gz.sha512
  | testing.go:1446: race detected during execution of test

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[cmacknz]

Fix is here #3685