[Translator] Reset all catalogues when adding resource to fallback locale (#7715, #7819)

Slightly refactored testAddResourceAfterTrans()
and replaced costly part with reset all catalogues

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7715, #7819
| License       | MIT
| Doc PR        |