I don't believe so. I only BRING the unit-TEST library for a project into a SOLUTION if I am modifying that project. Many times I am reusing existing libraries, such as a logging library, WITHOUT modifying it. In that case, there is no need to include the unit-tests for the logging library in the solution. Since we should never be modifying any code without first CREATING a test for it, there should be no risk of ever accidentally modifying code for which we have not included the test library in the solution.

I don't believe so. I only bring the unit-test library for a project into a solution if I am modifying that project. Many times I am reusing existing libraries, such as a logging library, without modifying it. In that case, there is no need to include the unit-tests for the logging library in the solution. Since we should never be modifying any code without first creating a test for it, there should be no risk of ever accidentally modifying code for which we have not included the test library in the solution.