Fix membership factories

The underlying association is `source` and we just add these `#project`
and `#group` methods that returns `source`.

That's why when doing something like:

```
member = build(:group_member, group: build(:group))
```

`member.group` is `nil`

spec/factories/group_members.rb

@@ -3,7 +3,7 @@
 FactoryBot.define do
   factory :group_member do
     access_level { GroupMember::OWNER }
-    group
+    source { association(:group) }
     user
 
     trait(:guest)     { access_level { GroupMember::GUEST } }

spec/factories/project_members.rb

@@ -3,7 +3,7 @@
 FactoryBot.define do
   factory :project_member do
     user
-    project
+    source { association(:project) }
     maintainer
 
     trait(:guest)     { access_level { ProjectMember::GUEST } }

spec/models/member_spec.rb

@@ -23,7 +23,8 @@ RSpec.describe Member do
     end
 
     context "when an invite email is provided" do
-      let(:member) { build(:project_member, invite_email: "user@example.com", user: nil) }
+      let_it_be(:project) { create(:project) }
+      let(:member) { build(:project_member, source: project, invite_email: "user@example.com", user: nil) }
 
       it "doesn't require a user" do
         expect(member).to be_valid
@@ -681,7 +682,7 @@ RSpec.describe Member do
       describe 'create member' do
         let!(:source) { create(source_type) }
 
-        subject { create(member_type, :guest, user: user, source_type => source) }
+        subject { create(member_type, :guest, user: user, source: source) }
 
         include_examples 'update highest role with exclusive lease'
       end