Use sum of subpaths instead of path.join in `no-deep-imports` lint rule (#50718)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50718

On Windows `path.join` returns path with separators unsupported by resolution mechanism. This change enforces the use of `/` separators in `no-deep-imports` rule tests.

Changelog:
[Internal]

Reviewed By: robhogan

Differential Revision: D73021185

fbshipit-source-id: d4799c01a5aef5b27fad961d774b58627115d213