I think that globals are not a problem when "you have a good [enough] understanding of the code you're writing." The problem is when code bases grow, references to globals can start to appear in lot of different places, and the exact use of a particular one can be hard to reason about. (Strictly talking about mutable global state here.)

As code bases grow, and developers come and go, eventually no one will have a "good [enough] understanding." Mutable global state is fundamentally hard to reason about since it can be changed at any time by any part of the program. When you first start out the codebase, you can just remember where all the usages are. But eventually that is not a good approach.

I consider the testing stuff orthogonal and muddying the issue. Mutable globals are hard to reason about, therefore they can make code hard to debug. Thus they should be avoided. No need to bring testing into the picture.