Fix remote mirror spec

These tests require a persisted project to work

spec/models/remote_mirror_spec.rb

@@ -442,16 +442,18 @@ RSpec.describe RemoteMirror, :mailer do
   end
 
   describe '#disabled?' do
+    let_it_be(:project) { create(:project, :repository) }
+
     subject { remote_mirror.disabled? }
 
     context 'when disabled' do
-      let(:remote_mirror) { build(:remote_mirror, enabled: false) }
+      let(:remote_mirror) { build(:remote_mirror, project: project, enabled: false) }
 
       it { is_expected.to be_truthy }
     end
 
     context 'when enabled' do
-      let(:remote_mirror) { build(:remote_mirror, enabled: true) }
+      let(:remote_mirror) { build(:remote_mirror, project: project, enabled: true) }
 
       it { is_expected.to be_falsy }
     end