[Discover] Migrate Karma test to functional tests where necessary

[kertal]

There are 3 Discover related test suites left in src/legacy/core_plugins/kibana/public/__tests__/discover
They are related to the legacy data table and should be analyzed whether they are still necessary, or already covered by functional tests. Where the tested use case isn't covered, functional tests should be created.

[kertal]

Closing this because all tests have been migrated in the meantime , last one here #72649