Synchronise between worker and test.

[howbazaar]

After adding more logging and running under the race detector, I was finally able to determine what was going on.

The test was looping around as soon as the worker signalled that the transactions were pruned, so the test clock was advanced another minute before the worker loop got around to call the next clock.After. Since we were waiting for a response every time we advanced the clock, it would fail because the worker already thought it was a minute after it should have been.

Fixes http://pad.lv/1623178

QA

Ran the test repeatedly with the race detector.

[howbazaar]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju