Add i18n-task rspec tests

[nschonni]

Notice during a bundle install

# Install default configuration:
cp $(bundle exec i18n-tasks gem-path)/templates/config/i18n-tasks.yml config/
# Add an RSpec for missing and unused keys:
cp $(bundle exec i18n-tasks gem-path)/templates/rspec/i18n_spec.rb spec/
# Or for minitest:
cp $(bundle exec i18n-tasks gem-path)/templates/minitest/i18n_test.rb test/
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The task part is already there, and there is the check-i18n.yml Action that does some of this and may be able to be dropped.

[nschonni]

Not sure if it would make sense to convert the checks in bundle exec rake repo:check_locales_files as well

[mjankowski]

I would prefer to leave these as-is (CLI task run in CI and locally at discretion) rather than default part of spec suite. They are pretty slow given the size of the translations.