Make unit tests more nuanced.

[chihuahua]

Today, our unit test is a large, monolithic test that

1. constructs a graph by passing messages to the panel script and
2. performs a massive object comparison:

https://github.com/google/audion/blob/master/tests/panel_test.js
This takes the unit out of unit testing. :(

Instead of doing a massive object property comparison, the test should assert for specific details such as the existence of various nodes as well as the specific connections.

Relatedly, we need to test more cases as noted in the TODO within that file. Making the test more concise is a first step that would enable that.

Writing tests is grungy, but important: As we develop more features, we must ensure that we preserve old behavior. I will contribute to this bug and expand test coverage - and anybody else is encouraged to as well.