'Sets up the nested hierarchy show below.
|     A     |
|   B   C   |
| D         |'

def _create_project_hierarchy(self):

    self.A = self.FakeProject()
    self.B = self.FakeProject(parent_id=self.A.id)
    self.C = self.FakeProject(parent_id=self.A.id)
    self.D = self.FakeProject(parent_id=self.B.id)
    self.B.subtree = {self.D.id: self.D.subtree}
    self.A.subtree = {self.B.id: self.B.subtree, self.C.id: self.C.subtree}
    self.A.parents = None
    self.B.parents = {self.A.id: None}
    self.C.parents = {self.A.id: None}
    self.D.parents = {self.B.id: self.B.parents}
    self.project_by_id = {self.A.id: self.A, self.B.id: self.B, self.C.id: self.C, self.D.id: self.D}
