Create source maps for Fantom lazily (#52786)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52786

Changelog: [internal]

It takes around 300ms to generate each source map file, and we really only need them if tests throw errors. Requesting source maps also increases contention to access the Metro server from each test worker.

This refactors the code so we only generate them in that case, which could save up to 20s in test execution time.

Reviewed By: rshest

Differential Revision: D78807672

fbshipit-source-id: af9f0f0377ddcf05014b5aca0b28db938dfb4ce2