Move test.js to a test/ directory.

Resolves https://github.com/DeNA/thrush/issues/5.

Raises istanbul's reported coverage to 100%.

```
=============================== Coverage summary ===============================
Statements   : 100% ( 48/48 )
Branches     : 100% ( 26/26 )
Functions    : 100% ( 13/13 )
Lines        : 100% ( 42/42 )
================================================================================
```