Usually codebases are organized into modules/classes/etc and most test suites can run specific test cases (or groups of tests, specific domains, etc).

I think if you can't do that in a codebase, then one has bigger organizational problems than the technology itself.

Also assigning many developers to work on the same set of files is a recipe for disaster (if said developers cannot get along or organize themselves) while working on the very same place/file.