SONARJAVA-5256 Add example of test using Spring extension of AssertJ

To show a false positive that was present before the fix on this branch.

java-checks-test-sources/default/pom.xml

@@ -224,6 +224,13 @@
       <type>jar</type>
       <scope>provided</scope>
     </dependency>
+    <!-- https://mvnrepository.com/artifact/io.spring.initializr/initializr-generator -->
+    <dependency>
+    <groupId>io.spring.initializr</groupId>
+    <artifactId>initializr-generator-test</artifactId>
+    <version>0.21.0</version>
+</dependency>
+
     <dependency>
       <groupId>org.springframework.security</groupId>
       <artifactId>spring-security-acl</artifactId>

java-checks-test-sources/default/src/test/java/checks/tests/AssertionsInTestsCheck/AssertJ.java

@@ -1,5 +1,6 @@
 package checks.tests.AssertionsInTestsCheck;
 
+import io.spring.initializr.generator.test.buildsystem.maven.MavenBuildAssert;
 import java.util.List;
 import org.assertj.core.api.AbstractListAssert;
 import org.assertj.core.api.Assertions;
@@ -9,9 +10,22 @@
 import org.assertj.core.api.SoftAssertions;
 import org.junit.Rule;
 import org.junit.Test;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.client.RestTemplate;
+
+import static com.google.gson.internal.TroubleshootingGuide.createUrl;
+import static org.mockito.Mockito.mock;
 
 public abstract class AssertJ {
 
+  @Test
+  public void usingExtendedAssert() {
+    ResponseEntity<String> response = mock(RestTemplate.class).getForEntity(createUrl("/pom.xml?packaging=war"),
+      String.class);
+    MavenBuildAssert pomAssert = new MavenBuildAssert(response.getBody());
+    pomAssert.hasPackaging("war");
+  }
+
   interface Visitor {
     void visit(Object object);
   }