TestRaceGetPut is flaky

This one: https://github.com/luci/gae/blob/master/impl/memory/race_test.go#L17

```
TestRaceGetPut (0.19s)
	race_test.go:63: Ran 314 inner functions
	race_test.go:65: value wrong value 100 v 99
```

Failed for me once when I was running `go test ./...` in luci/gae. Can't trigger it anymore :(