Building some command-line tests for blocks

[gvwilson]

Testing things by mousing and clicking is annoying, so I built a command-line widget to load the blocks code and generate some text (but not eval it, yet).

This depends on #39 and #40 - please merge those first.

1. npm run test in the project's root directory to run it.

2. tests/test.js reads ./index.html from standard input, finds all the blocks scripts, and eval's them.

3. In order for those eval's to work, tests/test.js defines simplified placeholders for the things the blocks need as they load up. (I figured this out by looking at "cannot find" error messages as blocks loaded and implementing each thing one by one.)

4. There is then an object Tests that has a bunch of tests, each written as a single function of no arguments with a meaningful name as its key. We loop over those, calling each function in turn; those functions create blocks, and we display the text they would generate.

This still requires manual inspection of output, but it's a step toward being able to check blocks without having to reload, drag, and click. The next steps will be to implement tests for the other blocks in generators/js/*.js and then add checks inside the tests to compare the actually-generated code with what we're expecting.