[c++] "The system cannot find the file specified" when running C++ program

For me, I didn't have my startup project set in Solution Explorer.

Go to Solution Explorer on the left of VS, right click your unit test project, and choose "set as startup project".

I had just ported my code to a new workspace, and forgot that when I opened the project in VS in the solution there, that I needed to re-set my startup project.