In fact when I configure check-in policies for unit test it is applied for the overall solution but I need a customized unit test check-in policy to be able to limit the scope of the policy only to a specific project, source file or smaller portions of code so that when I check in code changes of a specific project ,test check in policy will be applied for it and not for other projects in the solution.
Install the TFS Power Tools on the client computer then configure the Custom Path Policy to scope the check-in policy for unit test for a given path of your source control.