Code review comment for lp:~spiv/twisted/lineReceiver-bug-3277

I'm largely submitting this Launchpad merge proposal to experiment with
Launchpad, although I think the patch is good.)

To run the tests in this branch requires lp:testscenarios to be installed. I'm
not sure what to do about that, but the easy improvement in test coverage is
probably worth doing something to accomodate that.

This patch simply adds a TestCase that defines tests that both LineReceiver and
LineOnlyReceiver should meet. It turns out that actually there are some
inconsistencies in how these two classes behave for their supposedly common
features, so this patch also fixes the code so that the tests pass.

One unfixed issue I noticed is that LineOnlyReceiver does not implement the
IPushProducer interface, but LineReceiver does. I'm not sure if this is
intentional or not, and adding the feature to LineOnlyReceiver wasn't completely
trivial so I've punted on this issue and made that particular scenario skip.