You've probably named another class ProjectTest by mistake or purposely. I guess you were copy/pasting other test class, you've changed file name but forgot to rename class inside.
So when you run all unit tests PHPUnit is going through file tree and load all classes. If it meets two classes with the same name error "Fatal error: Cannot redeclare class X" occurs. SO solution for it would be to find second class with that name and fix it.
Otherwise if you keep two ProjectTest classes and that's how it should be (because you have to different Project classes under different packages), you can solve this problem by using namespaces:
Inside ProjectTest.php file before
next you have to replace all usages of methods and classes with it's name with '\' in front of it
For example if you have
you have to change it to
and so on.