Always clear runner strategy after each test case

Otherwise, we get different test results depending on the test order.

spec/controllers/execution_environments_controller_spec.rb

@@ -225,8 +225,6 @@ describe ExecutionEnvironmentsController do
     let(:runner_management_config) { {runner_management: {enabled: true, strategy: :poseidon}} }
 
     before do
-      # Ensure to reset the memorized helper
-      Runner.instance_variable_set :@strategy_class, nil
       allow(CodeOcean::Config).to receive(:new).with(:code_ocean).and_return(codeocean_config)
       allow(codeocean_config).to receive(:read).and_return(runner_management_config)
     end