test: Update chunk/cache/mock to have a GetKeys() function

[paul1r]

The new GetKeys() function will prevent a data race found.

What this PR does / why we need it:

Before the fix:

==================
WARNING: DATA RACE
Read at 0x00c000315140 by goroutine 111:
  github.com/grafana/loki/pkg/storage/chunk/fetcher.makeChunksFromMap()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher_test.go:344 +0x34
  github.com/grafana/loki/pkg/storage/chunk/fetcher.Test.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher_test.go:207 +0xf7c
  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

Previous write at 0x00c000315140 by goroutine 132:
  runtime.mapaccess2_faststr()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/runtime/map_faststr.go:108 +0x42c
  github.com/grafana/loki/pkg/storage/chunk/cache.(*mockCache).Store()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/cache/mock.go:28 +0x1bc
  github.com/grafana/loki/pkg/storage/chunk/fetcher.(*Fetcher).WriteBackCache()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher.go:318 +0xac4
  github.com/grafana/loki/pkg/storage/chunk/fetcher.(*Fetcher).asyncWriteBackCacheQueueProcessLoop()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher.go:136 +0x118
  github.com/grafana/loki/pkg/storage/chunk/fetcher.New.func2()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher.go:115 +0x34

Goroutine 111 (running) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x5e8
  github.com/grafana/loki/pkg/storage/chunk/fetcher.Test()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher_test.go:156 +0x2990
  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 132 (running) created at:
  github.com/grafana/loki/pkg/storage/chunk/fetcher.New()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher.go:115 +0x3bc
  github.com/grafana/loki/pkg/storage/chunk/fetcher.Test.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/fetcher/fetcher_test.go:196 +0xde4
  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:

fetcher % go test . -count=50 -race
ok  	github.com/grafana/loki/pkg/storage/chunk/fetcher	154.566s

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

[JordanRushing]

LGTM