I have a development database with one main schema and a bunch of developers who have their own data

What is the best way to set up this database so that the developer can unit test their code?

Here's the scenario:

there are public synonyms created for the objects in the main schema.

all of the stored objects have the AUTHID CURRENT_USER clause.

A developer copies a stored object into his/her own schema and modifies the code.  for the sake of this scenario, let's call this stored object, PKG1.

PKG1 can be called directly or it can be called by another stored object.  PKG1 will reference some other objects.

Currently, if PKG1 is called directly, it works.  But if PKG1 is called by another stored object which resided in the main schema, then it will call the PKG1 in the main schema, instead of the one in the developer's account.