More `BackendConnector.load` failure tests (#1812)
More `BackendConnector.load` failure tests (#1812)

* More failure tests

- Confirm that the callback is called even when some namespaces fail completely
- While writing these tests it became very clear that there is an inability to set the retry delay etc of `read` when using `BackendConnector.load`, as noted in #1809
- All of these tests pass with the code as-is :+1:

* Add a 10 namespace failure timing test

* test: remove only

Co-authored-by: Adriano Raiano <f000b1a7b08bda79c6c2a2c0cda21a0a0eededa8@users.noreply.github.com>
i18next/i18next,i18next/i18next,i18next/i18next,i18next/i18next