Setup approval tests to track significant behavior changes

The following 3 cases are tested:
- writing a complex model to a new file
- updating a complex model to an existing file containing a very minimal model
- updating a complex model to the file containing the complex model

pom.xml

@@ -48,6 +48,11 @@
             <version>2.9.1</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>com.approvaltests</groupId>
+            <artifactId>approvaltests</artifactId>
+            <version>22.3.2</version>
+        </dependency>
         <!-- needed during tests when running on Java9+ -->
         <dependency>
             <groupId>com.sun.xml.bind</groupId>

src/test/java/io/fabric8/maven/MavenTest.java

@@ -15,6 +15,7 @@
 
 import org.apache.maven.model.Model;
 import org.apache.maven.model.Scm;
+import org.approvaltests.Approvals;
 import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
@@ -120,6 +121,47 @@ void should_preserve_parent_relative_path(@TempDir Path tempDir) throws Exceptio
                 .isEqualTo("../../pom.xml");
     }
 
+    @Test
+    void should_save_full_pom_on_new_file(@TempDir Path tempDir) throws Exception {
+        URL resource = getClass().getResource("full-pom.xml");
+        Path basePom = Paths.get(resource.toURI());
+        Model model = Maven.readModel(new FileReader(basePom.toFile()));
+
+        Path newPom = tempDir.resolve("new-pom.xml");
+        Maven.writeModel(model, newPom);
+
+        Approvals.verify(Files.readString(newPom));
+    }
+
+    @Test
+    void should_save_full_pom_on_updated_full_file(@TempDir Path tempDir) throws Exception {
+        URL resource = getClass().getResource("full-pom.xml");
+        Path basePom = Paths.get(resource.toURI());
+        Model model = Maven.readModel(new FileReader(basePom.toFile()));
+
+        Path updatedPom = tempDir.resolve("updated-full-pom.xml");
+        Files.copy(basePom, updatedPom);
+        Maven.writeModel(model, updatedPom);
+
+        Approvals.verify(Files.readString(updatedPom));
+    }
+
+    @Test
+    void should_save_full_pom_on_updated_minimal_file(@TempDir Path tempDir) throws Exception {
+        URL resource = getClass().getResource("full-pom.xml");
+        Path basePom = Paths.get(resource.toURI());
+        Model model = Maven.readModel(new FileReader(basePom.toFile()));
+
+        Path updatedPom = tempDir.resolve("updated-minimal-pom.xml");
+        Files.writeString(updatedPom, "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
+                "<project xmlns=\"http://maven.apache.org/POM/4.0.0\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"\n"
+                +
+                "         xsi:schemaLocation=\"http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd\" />");
+        Maven.writeModel(model, updatedPom);
+
+        Approvals.verify(Files.readString(updatedPom));
+    }
+
     @Test
     void should_write_scm_tag(@TempDir Path tempDir) throws Exception {
         Path pomXml = tempDir.resolve("pom.xml");