Bug 991442 - Add coverage threshold to test-agent

https://bugzilla.mozilla.org/show_bug.cgi?id=991442
I didn't notice this line of code is too application-specific. Thanks for reminding me:)

The rule of our build system will put metadata.json into each app_dir, so we can assume the metadata.json is located in each app.

This application-specific code will be moved out of here and do it by passing argv from Makefile.