Add test to verify that coreModules are merged
Add test to verify that coreModules are merged

We already have code in place that makes sure that core modules are
merged if you specify two or more environments. We're about to move
coreModules into configuration instead, so we want to make sure we keep
that functionality.

trotzig/import-js,Galooshi/import-js,trotzig/import-js,trotzig/import-js