Add / Improve tests for tree goal

pom.xml

@@ -283,7 +283,13 @@ under the License.
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
-      <version>4.11</version>
+      <version>4.13</version>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.hamcrest</groupId>
+      <artifactId>hamcrest-all</artifactId>
+      <version>1.3</version>
       <scope>test</scope>
     </dependency>
     <dependency>

src/test/java/org/apache/maven/plugins/dependency/testUtils/stubs/CapturingLog.java

@@ -0,0 +1,171 @@
+package org.apache.maven.plugins.dependency.testUtils.stubs;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import org.apache.maven.plugin.logging.Log;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+public class CapturingLog implements Log
+{
+    private static final String NEWLINE = System.lineSeparator();
+
+    private StringBuffer buffer = new StringBuffer();
+
+    @Override
+    public boolean isDebugEnabled()
+    {
+        return true;
+    }
+
+    @Override
+    public void debug( CharSequence content )
+    {
+        buffer.append("DEBUG ");
+        buffer.append(content);
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void debug( CharSequence content, Throwable error )
+    {
+        buffer.append("DEBUG ");
+        buffer.append(content);
+        buffer.append(' ');
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void debug( Throwable error )
+    {
+        buffer.append("DEBUG ");
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public boolean isInfoEnabled()
+    {
+        return true;
+    }
+
+    @Override
+    public void info( CharSequence content )
+    {
+        buffer.append("INFO ");
+        buffer.append(content);
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void info( CharSequence content, Throwable error )
+    {
+        buffer.append("INFO ");
+        buffer.append(content);
+        buffer.append(' ');
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void info( Throwable error )
+    {
+        buffer.append("INFO ");
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public boolean isWarnEnabled() {
+        return true;
+    }
+
+    @Override
+    public void warn( CharSequence content )
+    {
+        buffer.append("WARNING ");
+        buffer.append(content);
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void warn( CharSequence content, Throwable error )
+    {
+        buffer.append("WARNING ");
+        buffer.append(content);
+        buffer.append(' ');
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void warn( Throwable error )
+    {
+        buffer.append("WARNING ");
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public boolean isErrorEnabled()
+    {
+        return true;
+    }
+
+    @Override
+    public void error( CharSequence content )
+    {
+        buffer.append("ERROR ");
+        buffer.append(content);
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void error( CharSequence content, Throwable error )
+    {
+        buffer.append("ERROR ");
+        buffer.append(content);
+        buffer.append(' ');
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    @Override
+    public void error( Throwable error )
+    {
+        buffer.append("ERROR ");
+        buffer.append( printThrowable( error ) );
+        buffer.append(NEWLINE);
+    }
+
+    private String printThrowable( Throwable error )
+    {
+        ByteArrayOutputStream output = new ByteArrayOutputStream();
+        error.printStackTrace( new PrintStream( output ) );
+        return output.toString();
+    }
+
+    public String getContent()
+    {
+        return this.buffer.toString();
+    }
+}