Add index.test.ts to each __tests__

[appsforartists]

It would be nice if our Mocha output was grouped by package. I wonder if we should put an index.test.ts in each __tests__ whose body looks basically like this:

describe('streams-dom', 
  () => {
    require('./SomeTest.test.ts');
  }
);

Manually adding require for each new test would be gross though. Maybe we should write a utility that requires all its siblings and run it in each index:

describe('streams-dom', requireAllSiblings);