Fix lagging unittests. The problem was a break in this code caused super-busy looping which massively lagged the process while exiting unittests. It's surprising that break here just breaks the select loop instead of the for loop. I audited the...

Summary:
...rest of the code for this pattern and it's clean.

Tobias, I think this means you can relax the skip slow tests change you
added.

Test Plan: go test

Reviewers: andybons, Tobias

Reviewed By: Tobias

Differential Revision: http://phabricator.cockroachdb.org/D65