fix: data race in distributor tests

[na--]

What this PR does / why we need it:

Tiny data race in the tests itself, without this fix the tests (when executed with -race) fail with:

WARNING: DATA RACE
Read at 0x00c00187a6a8 by goroutine 10064:
  github.com/grafana/loki/v3/pkg/distributor.TestDistributorPushToKafka.func3.1()
      github.com/grafana/loki/v3/pkg/distributor/distributor_test.go:586 +0x48
  github.com/stretchr/testify/assert.Eventually.func1()
      github.com/stretchr/testify@v1.10.0/assert/assertions.go:1949 +0x33

Previous write at 0x00c00187a6a8 by goroutine 9930:
  github.com/grafana/loki/v3/pkg/distributor.(*mockIngester).Push()
      github.com/grafana/loki/v3/pkg/distributor/distributor_test.go:1872 +0x3c4
  github.com/grafana/loki/v3/pkg/distributor.(*Distributor).sendStreamsErr()
      github.com/grafana/loki/v3/pkg/distributor/distributor.go:1035 +0x3af
...