fix: data race in ingester test

[na--]

What this PR does / why we need it:

Without these changes, the tests fail when executed with -race:

WARNING: DATA RACE
Write at 0x00c0126fd5b0 by goroutine 47612:
  github.com/grafana/loki/v3/pkg/ingester.(*readRingMock).GetAllHealthy()
      github.com/grafana/loki/v3/pkg/ingester/ingester_test.go:1541 +0x6e
  github.com/grafana/loki/v3/pkg/ingester.(*ownedStreamsIngesterStrategy).checkRingForChanges()
      github.com/grafana/loki/v3/pkg/ingester/recalculate_owned_streams.go:117 +0x77
  github.com/grafana/loki/v3/pkg/ingester.(*recalculateOwnedStreamsSvc).recalculate()
      github.com/grafana/loki/v3/pkg/ingester/recalculate_owned_streams.go:54 +0x2af
  github.com/grafana/loki/v3/pkg/ingester.(*recalculateOwnedStreamsSvc).iteration()
      github.com/grafana/loki/v3/pkg/ingester/recalculate_owned_streams.go:44 +0x33
  github.com/grafana/loki/v3/pkg/ingester.(*recalculateOwnedStreamsSvc).iteration-fm()
      <autogenerated>:1 +0x17
  github.com/grafana/loki/v3/pkg/ingester.Test_recalculateOwnedStreams_newRecalculateOwnedStreamsIngester.newRecalculateOwnedStreamsSvc.NewTimerService.func5()
      github.com/grafana/dskit@v0.0.0-20241007172036-53283a0f6b41/services/services.go:33 +0x1d8
  github.com/grafana/dskit/services.(*BasicService).main()
      github.com/grafana/dskit@v0.0.0-20241007172036-53283a0f6b41/services/basic_service.go:193 +0x3b7
  github.com/grafana/dskit/services.(*BasicService).StartAsync.func1.gowrap1()
      github.com/grafana/dskit@v0.0.0-20241007172036-53283a0f6b41/services/basic_service.go:122 +0x33

Previous read at 0x00c0126fd5b0 by goroutine 47613:
  github.com/grafana/loki/v3/pkg/ingester.Test_recalculateOwnedStreams_newRecalculateOwnedStreamsIngester.func1()
      github.com/grafana/loki/v3/pkg/ingester/recalculate_owned_streams_test.go:33 +0x37
  github.com/stretchr/testify/assert.Eventually.func1()
      github.com/stretchr/testify@v1.10.0/assert/assertions.go:1949 +0x33
...

[salvacorts]

👏 Thank you!