Fix an EE-only test order dependency

Timecop.freeze in a before block should always be paired with Timecop.return
in an after block

spec/models/remote_mirror_spec.rb

@@ -92,6 +92,10 @@ describe RemoteMirror do
       Timecop.freeze(Time.now)
     end
 
+    after do
+      Timecop.return
+    end
+
     context 'with remote mirroring enabled' do
       it 'schedules a RepositoryUpdateRemoteMirrorWorker to run within a certain backoff delay' do
         expect(RepositoryUpdateRemoteMirrorWorker).to receive(:perform_in).with(RemoteMirror::BACKOFF_DELAY, remote_mirror.id, Time.now)
@@ -126,6 +130,10 @@ describe RemoteMirror do
       remote_mirror.update_attributes(last_update_started_at: Time.now)
     end
 
+    after do
+      Timecop.return
+    end
+
     context 'when remote mirror does not have status failed' do
       it 'returns true when last update started after the timestamp' do
         expect(remote_mirror.updated_since?(timestamp)).to be true