chore: ☂️ Migrate GeneratePackageListTaskTest.kt to AssertJ (#45790)

Summary:
Issue: https://github.com/facebook/react-native/issues/45596

## Changelog:
[INTERNAL] [CHANGED] - Migrate `GeneratePackageListTaskTest.kt` to AssertJ testing library

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Pull Request resolved: https://github.com/facebook/react-native/pull/45790

Test Plan: Run `./gradlew -p packages/gradle-plugin test`

Reviewed By: sammy-SC

Differential Revision: D60382287

Pulled By: cortinico

fbshipit-source-id: 338c771db9d407f0d83e1c62a8c13ac26d898926