Add tests for connection lines starting from pins

[amstan]

In https://github.com/google/pcbdl/commit/d53d183b1eb44a23925aa264589c4f6681e8c1cf I added to the functionality that allows a Pin to be used instead of a Net for a lot of cases.

Should write tests for that.

Perhaps make a Connectable protocol: something that implements .connect, .__lshift__ and .__rshift__. Pin and Net would both support it.

[amstan]

And this is what happens when there's no tests for it.

Instead of defining rshift, we defined an lshift twice: https://github.com/google/pcbdl/blob/master/pcbdl/base.py#L370

Ty @crcollins for the discovery.