Why this change?
We have been bitten by bugs where tests are not catching missing
interpolate argument in our client side code because the JavaScript
tests are also using `I18n.translate` to assert that the right message
is shown. Before this change, `I18n.interpolate` will just replace the
missing interpolation argument in the final translation with some
placeholder. As a result, we ended up comparing a broken translation
with another broken translation in the test environment.
Why does this change do?
This change introduces the `I18n.testing` property which when set to
`true` will cause `I18n.translate` to throw an error when an interpolate
argument is missing. With this commit, we also set `I18n.testing = true`
when running qunit acceptance test.
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