[TAV-485] Better tests for the worker code.

Use flexmock to stub parts of the Taverna Server client libraries so the worker
code can be tested within the unit tests.
Set Taverna Server config in the tests for Travis.

There are no defaults for these settings as they are server names and passwords,
etc.
[TAV-485] Ensure run is backed-off before starting.

Simulate the server telling the run to wait.
[TAV-485] Ensure run is backed-off before creation.

Simulate the server telling the worker it is full.
[TAV-485] Test a failing run.
[TAV-485] Test a cancelled run.
Remove worker integration test file.

No longer needed as we mock things in the unit tests now.