test(ngMessages): use strict-di for ngMessages tests

[caitp]

This will hopefully prevent issues similar to the one fixed by 63b100c

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#7477)

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!