Andrew Bancroft tested and analyzed the behavior of extensions in Swift.
While his findings aren’t utterly surprising, having them summed up in this nicely done article certainly helps.

In a nutshell:

• extensions in the same file as their source can access even privat members (attributes and methods, that is)
• extensions in different files can access internal and public members – but only if they are in the same module
• if extensions are in a different module, they merely have access to public members
• classes with public or internal visibility on themselves expose their members as internal by default
• classes with private visibility on themselves expose their members as private
• extensions themselves don’t have to be public for public members to work (unlike classes)

This has consequences for your tests: they reside in a separate module, so they can only access public members of your classes.

If you extend classes from static libraries, the same holds true. Different module, only public accessors.