disable test with command line engine

Author: nidi3

graphviz-java/src/test/java/guru/nidi/graphviz/model/ReadmeTest.java

@@ -172,6 +172,7 @@ void ex6() throws IOException {
     }
 
     @Test
+    @Disabled("Because there's no graphviz installed in the build server")
     void ex7() throws IOException {
         //## img
         Graphviz.useEngine(new GraphvizCmdLineEngine());
@@ -184,7 +185,6 @@ void ex7() throws IOException {
     @Test
     void ex8() throws IOException {
         //## image
-        Graphviz.useEngine(new GraphvizV8Engine());
         Graphviz g = Graphviz.fromGraph(graph()
                 .with(node(" ").with(Size.std().margin(.8, .7), Image.of("graphviz.png"))));
         g.basedir(new File("example")).render(Format.PNG).toFile(new File("example/ex8.png"));