Add test for docker agent with Closure param

src/test/groovy/com/lesfurets/jenkins/unit/declarative/TestDockerAgentInStep.groovy

@@ -0,0 +1,19 @@
+package com.lesfurets.jenkins.unit.declarative
+
+import org.junit.Before
+import org.junit.Test
+
+class TestDockerAgentInStep extends DeclarativePipelineTest {
+
+    @Before
+    @Override
+    void setUp() throws Exception {
+        super.setUp()
+    }
+
+    @Test
+    void test_example() {
+        runScript("src/test/jenkins/jenkinsfiles/Docker_agentInStep_JenkinsFile")
+        assertJobStatusSuccess()
+    }
+}

src/test/jenkins/jenkinsfiles/Docker_agentInStep_JenkinsFile

@@ -0,0 +1,11 @@
+pipeline {
+
+    stages {
+        stage('Maven Build') {
+            agent { docker { image 'maven' } }
+            steps {
+                sh 'mvn install'
+            }
+        }
+    }
+}