Bug 1024791 - Scan the add-on for tests

You could leverage a lot of utilities present in `util/sequence` to create lazy iterator instead, I also find it little easier to read give that each function does one simple operation ;)

```js
const { map, filter, fromEnumerator } = require("sdk/util/sequence");

const toFile = x => x.QueryInterface(Ci.nsIFile);
const isTestFile = ({leafName}) => leafName.substr(0, 5) == "test-" && leafName.substr(-3, 3) == ".js";
const getFileURI = x => ios.newFileURI(x).spec;

const getDirectoryEntries = file => map(toFile, fromEnumerator(file.directoryEntries));
const getTestFiles = directory => filter(isTestFile, getDirectoryEntries(directory));
const getTestURIs = directory => map(getFileURI, getTestFiles(directory));
```