Add execute_command method to runner.rb

* This is now used by the score and test runs
* This also re-enables the interactive shell for execution environments

spec/models/execution_environment_spec.rb

@@ -149,10 +149,12 @@ describe ExecutionEnvironment do
 
     before { allow(DockerClient).to receive(:find_image_by_tag).and_return(Object.new) }
 
-    it 'instantiates a Docker client' do
-      expect(DockerClient).to receive(:new).with(execution_environment: execution_environment).and_call_original
-      allow_any_instance_of(DockerClient).to receive(:execute_arbitrary_command).and_return({})
+    it 'instantiates a Runner' do
+      runner = instance_double 'runner'
+      allow(Runner).to receive(:for).with(execution_environment.author, execution_environment).and_return runner
+      allow(runner).to receive(:execute_command).and_return({})
       working_docker_image?
+      expect(runner).to have_received(:execute_command).once
     end
 
     it 'executes the validation command' do

spec/models/runner_spec.rb

@@ -247,7 +247,7 @@ describe Runner do
       before { allow(strategy_class).to receive(:request_from_management).and_return(nil) }
 
       it 'raises an error' do
-        expect { described_class.for(user, exercise) }.to raise_error(Runner::Error::Unknown, /could not be saved/)
+        expect { described_class.for(user, exercise.execution_environment) }.to raise_error(Runner::Error::Unknown, /could not be saved/)
       end
     end
 
@@ -255,12 +255,12 @@ describe Runner do
       let!(:existing_runner) { FactoryBot.create(:runner, user: user, execution_environment: exercise.execution_environment) }
 
       it 'returns the existing runner' do
-        new_runner = described_class.for(user, exercise)
+        new_runner = described_class.for(user, exercise.execution_environment)
         expect(new_runner).to eq(existing_runner)
       end
 
       it 'sets the strategy' do
-        runner = described_class.for(user, exercise)
+        runner = described_class.for(user, exercise.execution_environment)
         expect(runner.strategy).to be_present
       end
     end
@@ -269,7 +269,7 @@ describe Runner do
       before { allow(strategy_class).to receive(:request_from_management).and_return(runner_id) }
 
       it 'returns a new runner' do
-        runner = described_class.for(user, exercise)
+        runner = described_class.for(user, exercise.execution_environment)
         expect(runner).to be_valid
       end
     end