Always clear runner strategy after each test case

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

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