test: Add synchronization for fakeTailer responses

[paul1r]

What this PR does / why we need it:
Data race found. Before the fix:

go test -race -count=10 . -run Test_StructuredMetadata 
==================
WARNING: DATA RACE
Read at 0x00c000c225d0 by goroutine 113:
  github.com/grafana/loki/pkg/ingester.(*fakeTailServer).GetResponses()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:164 +0x30
  github.com/grafana/loki/pkg/ingester.Test_StructuredMetadata.func1.2()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:326 +0x40
  github.com/stretchr/testify/assert.Eventually.func1()
      /Users/progers/dev/src/github.com/grafana/loki/vendor/github.com/stretchr/testify/assert/assertions.go:1852 +0x38

Previous write at 0x00c000c225d0 by goroutine 110:
  github.com/grafana/loki/pkg/ingester.(*fakeTailServer).Send()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:156 +0x138
  github.com/grafana/loki/pkg/ingester.(*tailer).loop()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer.go:105 +0x214
  github.com/grafana/loki/pkg/ingester.Test_StructuredMetadata.func1.1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:318 +0x34

Goroutine 113 (running) created at:
  github.com/stretchr/testify/assert.Eventually()
      /Users/progers/dev/src/github.com/grafana/loki/vendor/github.com/stretchr/testify/assert/assertions.go:1852 +0x2b8
  github.com/stretchr/testify/require.Eventually()
      /Users/progers/dev/src/github.com/grafana/loki/vendor/github.com/stretchr/testify/require/require.go:401 +0x90
  github.com/grafana/loki/pkg/ingester.Test_StructuredMetadata.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:325 +0x30c
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40

Goroutine 110 (running) created at:
  github.com/grafana/loki/pkg/ingester.Test_StructuredMetadata.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/ingester/tailer_test.go:317 +0x1e4
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40
==================

After the fix:

go test -race -count=10 . -run Test_StructuredMetadata
ok  	github.com/grafana/loki/pkg/ingester	21.665s

Which issue(s) this PR fixes:
Relates to: #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[trevorwhitney]

lgtm

[trevorwhitney]

nit: I don't think we need the *fakeTailerServer reciever here