Remove warning "No translation for appId "${appId}" have been registered"

[ShGKme]

In 1.x translations registry was initialized in OC.L10N. The initialization includes defining global variables on script load: https://github.com/nextcloud/server/blob/352bd7ecea488201e4162e4e31a0cf9770cf20a0/core/src/OC/l10n-registry.js#L24-L26

// server/core/src/OC/l10n-registry.js 
window._oc_l10n_registry_translations = window._oc_l10n_registry_translations || {}
window._oc_l10n_registry_plural_functions = window._oc_l10n_registry_plural_functions || {}

After an update @nextcloud/l10n@2.0.0, the lib becomes independent from the server's OC.L10N. But it still required window._oc_l10n_registry_translations to be initialized and warned No OC L10N registry found.

In #556 it was fixed by defining window._oc_l10n_registry_translations on the first registration. But it also added a warning if there is no translation for an app No translation for appId "${appId}" have been registered.

The problem is that English translations often are not supposed to be registered. Instead, the translation keys are used. It results in tons of useless warnings in the console and impacts DX.

This PR:

• Removes the warning at all and updates tests
• Add console.warn mock in gettext tests to make testing silent

[ChristophWurst]

🧹 ✨