How to load module include files in .test file?
You have a flaw in your code, module_load_include works by including files relative to the module base dir, so you should do

module_load_include('inc', 'password_policy', 'constraints /constraint_uppercase');


to include password_policy/constraints/constraint_uppercase.inc

When you do

module_load_include('inc', 'password_policy', 'constraint_uppercase');


You instead try to include password_policy/constraint_uppercase.inc which doesn't exist.