[IMP] project: prevent test_delete_project_with_task to be run several times

Prior to this commit, `test_delete_project_with_task` is part of `TestProjectCommon`
which is inherited in several tests. Because of that, `test_delete_project_with_task`
is run several times, which is not relevant.

This commit moves the tests into a new class in order to prevent it to be run
several times.

task-2845729

addons/project/tests/test_project_base.py

@@ -94,6 +94,9 @@ class TestProjectCommon(TransactionCase):
         self.env['mail.thread'].message_process(model, mail)
         return self.env[target_model].search([(target_field, '=', subject)])
 
+
+class TestProjectBase(TestProjectCommon):
+
     def test_delete_project_with_tasks(self):
         """User should never be able to delete a project with tasks"""