arcyd-tester: refactor, extract _Worker

Test Plan:
$ ./precommit.sh
arcyd-tester: refactor, extract _CommandWithWor...

Test Plan:
$ ./precommit.sh
arcyd-tester: _Worker supports barc

We'll want to query the state of reviews, 'barc' is a convenient way to
determine the status of reviews just by watching git refs.

Expose a 'barc' property on _Worker for calling through to 'barc'

Test Plan:
$ ./precommit.sh