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

spec/lib/runner/strategy/poseidon_spec.rb

@@ -13,8 +13,6 @@ describe Runner::Strategy::Poseidon do
   let(:runner_management_config) { {runner_management: {enabled: true, strategy: :poseidon, url: 'https://runners.example.org', unused_runner_expiration_time: 180}} }
 
   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

spec/models/runner_spec.rb

@@ -34,8 +34,6 @@ describe Runner do
       let(:runner_management_config) { {runner_management: {enabled: true, strategy:}} }
 
       before do
-        # Ensure to reset the memorized helper
-        described_class.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

spec/support/runner.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+def reset_runner_strategy
+  Runner.instance_variable_set :@strategy_class, nil
+  Runner.instance_variable_set :@management_active, nil
+end
+
+RSpec.configure do |config|
+  # When starting the application, the environment is initialized with the default runner strategy:
+  # `Runner.strategy_class.initialize_environment` is called in `config/application.rb`.
+  config.before(:suite) do
+    reset_runner_strategy
+  end
+
+  # After each test, we reset the memorized runner strategy to the default (similar to the database cleaner).
+  config.append_after do
+    reset_runner_strategy
+  end
+end