[Unzyme] `src/core/packages/notifications`

[afharo]

Migrate Core UI tests from enzyme to react testing library for the following files:

• https://github.com/elastic/kibana/blob/main/src/core/packages/notifications/browser-internal/src/toasts/deduplicate_toasts.test.tsx
• https://github.com/elastic/kibana/blob/main/src/core/packages/notifications/browser-internal/src/toasts/error_toast.test.tsx

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)