Testing and package management aren't mutually exclusive. If you're in a monorepo, all your code evolves in lockstep. A core tenet of versioned package management is to get reproducible builds that only need to break once you choose to evolve forward in time (i.e. upgrade).
Historically, most languages (C, C++, pre-Maven Java) haven't had package management at all, and so dependencies have typically been managed by vendoring the code (or JAR files). JAR files worked okay, but vendoring incurs maintenance overhead that isn't acceptable in today's environment. git submodules are theoretically a solution, but also high-maintenance.
Historically, most languages (C, C++, pre-Maven Java) haven't had package management at all, and so dependencies have typically been managed by vendoring the code (or JAR files). JAR files worked okay, but vendoring incurs maintenance overhead that isn't acceptable in today's environment. git submodules are theoretically a solution, but also high-maintenance.