Migrate to _tests.js files in the tests/ dir

Moved all the arena tests to arena_tests.js
Move test.js to /tests
Moved arena_tests.js to /tests

Things were getting a little crowded in arena.js and I've got all the
initial tests written, so now I can punt them off to another file.

Also updated boids.html to reflect the changes. I imagine I'll have to
make this dynamic at some point, as I'm going to screw up the code
loading eventually.