As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.
This commit also introduces an eslint rule to prevent importing from
the old I18n path.
For themes/plugins, the old 'i18n' name remains functional.
1. `test()` and `render()` instead of `componentTest()`
2. Angle brackets
3. `strictEqual()`/`true()`/`false()` assertions
This removes all remaining uses of `componentTest` from core
