worker/terminationworker: fix test hang

[davecheney]

Fixes LP 1588135

Fix a race on the ownership of the abort signal.

The TestSignal test attempts to see if sending SIGABRT to the test
binary itself will cause the worker to unblock itself and exit as
exited. However, as the original author discovered there is a race
between sending the signal and the worker spawned from NewWorker
registering a handler to catch SIGABRT. If the handler was not registered
in time the SIGABRT would cause the test process to exit.

To fix this problem they set up a SIGABRT handler in SetUpTest to ensure
that SIGABRT was always handled. However this meant that if NewWorker's
handler was not registered in time to catch the signal, it would be
delivered to the channel registered by SetUpTest, causing the test binary
to hang.

The fix is to remove the SetUpTest workaround and ensure the handler is
always registered before spawning the loop goroutine so the caller is
assured that once NewWorker has returned, the handler is in place and it
is safe to send the signal.

This PR also removes unnecessary use of testing.BaseSuite.

(Review request: http://reviews.vapour.ws/r/4975/)

[davecheney]

$$fixes-1588135$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7972

[davecheney]

$$fixes-1588135$$

[jujubot]

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