Modify tests to load `build/msg/en.js` instead of `msg/messages.js`

[cpcallen]

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Description

Most of the tests (including notably the playgrounds) have in the past loaded message via <script src="path/to/msg/messages.js">, but—although this works—messages.js is not intended to be used this way. They should instead be using the built en.js langfile.

For now it is probably preferable to use the un-UMD-wrapped version in build/ instead of the wrapped version in dist/ just to avoid having to run the package gulp task in order to run tests / load the playground.

Additional information

This task was broken out of issue #6403 which is otherwise complete.