Include shared entry file in tests for real coverage

[dennisgl]

A common issue with nyc/istanbul is the coverage only covers as far as the test entry points reach but not the whole bundle, giving a false sense of high coverage. Here's a similar issue describing the problem: gotwarlost/istanbul#112

--include-all-sources in Istanbul v1, now --all in Istanbul v2/nyc extends the coverage area to the whole bundle. However, our bundles including test bundles are tree-shaked, therefore the option won't save the situation.

The changes here include shared entry point(currently src/main.js to test entry points passing to multi-entry-loader

Open for comments, given this basically eliminates tree-shaking for test bundles.

[ganemone]

makes sense to me. I imagine we probably don't care about tree shaking for test builds

[dennisgl]

!merge