Bug 1024791 - Scan the add-on for tests

After few scans through this function my conclusion is that it takes a file, presumably directory and then recursively walks it collecting all `test/test-*.js` entries. I would really appreciate some comment explaining this cause otherwise it's kind of hard to see what is it doing. I would also prefer expressing it more simply by use of `util/sequence`:

```js
const isDirectory = x = x.isDirectory();
const getTestEntries = directory => mapcat(entry =>
  entry.leafName == "tests" ? getTestURIs(entry) : getTestEntries(entry),
  filter(isDirectory, getDirectoryEntries(directory)));
```