Bug 991442 - Add coverage threshold to test-agent

except that you can't build a profile anymore once you do this.

```
System JS : ERROR resource://gre/modules/commonjs/toolkit/loader.js -> file:///home/julien/travail/git/gaia/build/utils-xpc.js:63 - Error: "external" property in metadata.json is required since Firefox OS 2.1, please add it into metadata.json and update preload.py if you use this script to perload your apps. If you created metadata.json for non-external apps, please set "external" to false. your metadata.json is in /home/julien/travail/git/gaia/apps/bluetooth
```

So either you modify the build system so that it can ignore this error, or we choose another file name (unit.json for example).