Extract test fixtures to separate header (#55056)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55056

Refactors InspectorPackagerConnectionTest.cpp to extract the test fixtures into a separate header file. This allows us to split `InspectorPackagerConnectionTest.cpp` into multiple files as we add more test cases, which is particularly helpful for LLMs writing tests.

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D90174645

fbshipit-source-id: aa2c2a706391060660adb13a915b57bcea539e2d