Fix flaky test in cluster_agents_spec.rb

Two of the three tokens had `nil` `last_used_at` dates, and the database
can return those in any order. To fix the spec, we assign one of the
`nil` values a date. The order also needs to be sorted most recently
used, with `nil` at the end.

ee/spec/requests/api/graphql/project/cluster_agents_spec.rb

@@ -51,7 +51,7 @@ RSpec.describe 'Project.cluster_agents' do
 
   context 'selecting tokens' do
     let_it_be(:token_1) { create(:cluster_agent_token, agent: agents.first) }
-    let_it_be(:token_2) { create(:cluster_agent_token, agent: agents.second) }
+    let_it_be(:token_2) { create(:cluster_agent_token, agent: agents.second, last_used_at: 3.days.ago) }
     let_it_be(:token_3) { create(:cluster_agent_token, agent: agents.second, last_used_at: 2.days.ago) }
 
     let(:cluster_agents_fields) { [:id, query_nodes(:tokens, of: 'ClusterAgentToken')] }
@@ -62,9 +62,9 @@ RSpec.describe 'Project.cluster_agents' do
       tokens = graphql_data_at(:project, :cluster_agents, :nodes, :tokens, :nodes)
 
       expect(tokens).to match([
-        a_hash_including('id' => global_id_of(token_1)),
         a_hash_including('id' => global_id_of(token_3)),
-        a_hash_including('id' => global_id_of(token_2))
+        a_hash_including('id' => global_id_of(token_2)),
+        a_hash_including('id' => global_id_of(token_1))
       ])
     end