You need to go to the project settings with tests and in VC++ Directories , in the field Include Directories add the directory with the headers you need. It will look something like this: ../ProjectToTest/Headers - it is better to set a relative path, otherwise everything will break if moved to another place.
But you need to understand that the headlines are not the only problem; if the project that you want to test has cpp files, then it should be the library that you connect to the test project. Otherwise, connecting the headers will cause you to get a linker error.
And a link to MSDN where you can learn how to add a link to a neighboring project so that the lib automatically used by the current project: Adding references in Visual C ++ projects