Add functionality marge for build extensions section

mixin-maven-plugin/src/test/java/com/playtika/maven/plugins/MixinModelMergeBuildExtensions.java

@@ -0,0 +1,75 @@
+package com.playtika.maven.plugins;
+
+import java.util.List;
+
+import org.apache.maven.model.Build;
+import org.apache.maven.model.Extension;
+import org.apache.maven.model.Model;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+
+public class MixinModelMergeBuildExtensions {
+
+    private MixinModelMerger merger;
+    private Model buildExtensions, emptyBuildExtensions;
+
+    @BeforeEach
+    public void setup() {
+        merger = new MixinModelMerger();
+        buildExtensions = new Model();
+        buildExtensions.setBuild(new Build());
+        Extension extension = new Extension();
+        extension.setGroupId("com.test");
+        extension.setArtifactId("test-magre-extension");
+        extension.setVersion("1.0.0");
+        buildExtensions.getBuild().getExtensions().add(extension);
+
+        emptyBuildExtensions = new Model();
+        emptyBuildExtensions.setBuild(new Build());
+    }
+
+    @Test
+    public void testEmptyTarget() {
+        List<Extension> origExtensions = buildExtensions.getBuild().getExtensions();
+        assertNotNull(origExtensions);
+        assertEquals(origExtensions.size(), 1);
+        merger.mergeBuildExtensions(emptyBuildExtensions, buildExtensions);
+        assertEquals(origExtensions, buildExtensions.getBuild().getExtensions());
+    }
+
+    @Test
+    public void testEmptySrc() {
+        merger.mergeScm(buildExtensions, emptyBuildExtensions);
+        assertNotNull(emptyBuildExtensions.getBuild().getExtensions());
+        assertEquals(emptyBuildExtensions.getBuild().getExtensions().size(), 0);
+
+        assertNotNull(buildExtensions.getBuild().getExtensions());
+        assertEquals(buildExtensions.getBuild().getExtensions().size(), 1);
+    }
+
+    @Test
+    public void testMageFromSrc() {
+        emptyBuildExtensions = new Model();
+        emptyBuildExtensions.setBuild(new Build());
+        Extension extension = new Extension();
+        extension.setGroupId("com.test2");
+        extension.setArtifactId("test-magre-extension-v2");
+        extension.setVersion("2.0.0");
+        emptyBuildExtensions.getBuild().getExtensions().add(extension);
+
+        assertNotNull(buildExtensions.getBuild().getExtensions());
+        assertEquals(1, buildExtensions.getBuild().getExtensions().size());
+
+        assertNotNull(emptyBuildExtensions.getBuild().getExtensions());
+        assertEquals(1, emptyBuildExtensions.getBuild().getExtensions().size());
+
+        merger.mergeBuildExtensions(buildExtensions, emptyBuildExtensions);
+
+        assertNotNull(buildExtensions.getBuild().getExtensions());
+        assertEquals(2, buildExtensions.getBuild().getExtensions().size());
+    }
+}