Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
> Oh, I meant build system will read metadata.json with .../app_dir/metadata.json. If there is no such file exist, build system will skip.

But... if the file exist but has no "external" property, the build system throws.

Just try it with your patch !! :)