Fix failing test in spec/workers/post_receive_spec.rb

This is what was happening before:

1. `Project#set_timestamps_for_create` was called at creation time
and set the `last_activity_at` and `last_repository_updated_at`
to the current timestamp T.

2. The test ran `PostReceive#perform`, which then called
`PostReceive#process_wiki_changes`. If less than 500 milliseconds
elapsed since T, then the update would just set the timestamp to T.

To fix this problem, we can just use Timecop to ensure at least
one second has elapsed after attempting to process changes.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8871

spec/workers/post_receive_spec.rb

@@ -141,13 +141,20 @@ describe PostReceive do
     let(:gl_repository) { "wiki-#{project.id}" }
 
     it 'updates project activity' do
-      described_class.new.perform(gl_repository, key_id, base64_changes)
+      # Force Project#set_timestamps_for_create to initialize timestamps
+      project
 
-      expect { project.reload }
-        .to change(project, :last_activity_at)
+      # MySQL drops milliseconds in the timestamps, so advance at least
+      # a second to ensure we see changes.
+      Timecop.freeze(1.second.from_now) do
+        expect do
+          described_class.new.perform(gl_repository, key_id, base64_changes)
+          project.reload
+        end.to change(project, :last_activity_at)
            .and change(project, :last_repository_updated_at)
       end
     end
+  end
 
   context "webhook" do
     it "fetches the correct project" do