Test flake in chunk package

[bboreham]

A couple of CI runs have errored after a 60-second timeout on github.com/cortexproject/cortex/pkg/chunk:

https://circleci.com/gh/cortexproject/cortex/5115
https://circleci.com/gh/cortexproject/cortex/5138

Looking at other recent timings, it seems likely that the test run takes a long time every run, and randomly happens to go over 60 seconds.

https://circleci.com/gh/cortexproject/cortex/5090 60.666s
https://circleci.com/gh/cortexproject/cortex/5097 45.666s
https://circleci.com/gh/cortexproject/cortex/5100 48.006s
https://circleci.com/gh/cortexproject/cortex/5129 44.467s