Fix cleanup rake spec by adding clear_pool in after do block

spec/lib/gitlab/redis/sessions_spec.rb

@@ -10,6 +10,10 @@ RSpec.describe Gitlab::Redis::Sessions do
       clear_pool
     end
 
+    after do
+      clear_pool
+    end
+
     context 'when redis.sessions configuration is not provided' do
       it 'uses ::Redis instance' do
         expect(described_class).to receive(:config_fallback?).and_return(true)