Support in-repo engine coverage #86

Ember-engines allow use in-repo style of engine location, in this case testing results of main app includes testing results of engine. But code coverage output, unlike tap file, not includes coverage results for engine.

Steps to reproduce:

• git clone https://github.com/Nuzsk/ember-engine-coverage-test.git
• cd ember-engine-coverage-test && npm install && bower install
• COVERAGE=true ember test > test-results.tap
• cat test-results.tap | grep test-me
• cat coverage/coverage-summary.json | grep test-me