[MJAVADOC-642] Make offline mode configurable (#238)

Turns out that `mvn deploy` and `mvn clean deploy` do different things
with settings. Explicitly add settings to the test-javadoc-test-plugin
config, otherwise it throws a NPE when running `mvn deploy`.
diff --git a/src/test/resources/unit/test-javadoc-test/test-javadoc-test-plugin-config.xml b/src/test/resources/unit/test-javadoc-test/test-javadoc-test-plugin-config.xml
index 8155329..bf0b5ff 100644
--- a/src/test/resources/unit/test-javadoc-test/test-javadoc-test-plugin-config.xml
+++ b/src/test/resources/unit/test-javadoc-test/test-javadoc-test-plugin-config.xml
@@ -55,6 +55,7 @@
           <reactorProjects>
             <project implementation="org.apache.maven.plugins.javadoc.stubs.TestJavadocMavenProjectStub"/>
           </reactorProjects>
+          <settings implementation="org.apache.maven.settings.Settings"/>
         </configuration>
       </plugin>
     </plugins>