Create some basic ingester flushing tests

[juliusv]

Bugs around that logic have bitten us a couple of times now, so it'd be good to have at least some basic tests.

[tomwilkie]

We have https://github.com/weaveworks/cortex/blob/master/ingester/ingester_test.go#L77, which has proven quite useful. I think you wrote it @juliusv!