fix: data race in distributor tests (#15423)

Author: na--

pkg/distributor/distributor_test.go

@@ -583,6 +583,8 @@ func TestDistributorPushToKafka(t *testing.T) {
 		require.Equal(t, 1, len(ingesters[0].pushed))
 		require.Equal(t, 1, len(ingesters[1].pushed))
 		require.Eventually(t, func() bool {
+			ingesters[2].mu.Lock()
+			defer ingesters[2].mu.Unlock()
 			return len(ingesters[2].pushed) == 1
 		}, time.Second, 10*time.Millisecond)
 	})