Cleanup Modal test code (#46603)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46603

Was debugging this test for the mounting instruction changes, and found some opportunities to clean this up.

Changelog: [Internal]

Reviewed By: cortinico

Differential Revision: D63258342

fbshipit-source-id: f7e6be58474f112993232ce5859ccb160f050ae8