This is exactly why I only ever test at the edges of a project, using mock systems that are as realistic as possible given time and resource constraints. The industry has an unhealthy obsession with ultrafast, ultra-unrealistic, low level unit tests that tightly couple to implementation details rather than behavior.

I don't think this is over-engineering though. Even if unit tests didn't warp designs they'd be a bad idea. I think it's just bad engineering based upon stupid dogma spread by the likes of J.D. Rainsberger and Uncle Bob.