Expand Fantom integration tests for FlatList coverage (#55930)

Summary:
Pull Request resolved: #55930

Expand FlatList Fantom tests to improve FlatList.js code coverage from 44% to 62% line coverage.

Changelog: [Internal]

Reviewed By: zeyap

Differential Revision: D94361042

fbshipit-source-id: 4923886eb7adfa5e5513e1e0a58c6b6f6ff72a2a

Author: sammy-SC