Add dependency tests

- add `testGraphviz`: request `version` as AsciiArt and search for
  "Installation seems OK. File generation OK"
- irgnore `testGraphviz` inside github workflow tests except for the
  docker container tests, since only in the docker container case
  graphviz is available
- add `testMonacoEditorWebJar`: request `loader.js` and check if the
  server answers with status code 200
- add a note inside the pom where the webjar versions are hard coded
  inside the project

src/test/java/net/sourceforge/plantuml/servlet/TestDependencies.java

@@ -0,0 +1,53 @@
+package net.sourceforge.plantuml.servlet;
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.URL;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+
+import net.sourceforge.plantuml.servlet.utils.TestUtils;
+import net.sourceforge.plantuml.servlet.utils.WebappTestCase;
+
+
+public class TestDependencies extends WebappTestCase {
+
+    /**
+     * Verifies that Graphviz is installed and can be found
+     */
+    @Test
+    @Tag("graphviz-test")
+    public void testGraphviz() throws IOException {
+        final URL url = new URL(getServerUrl() + "/txt/" + TestUtils.VERSION);
+        final HttpURLConnection conn = (HttpURLConnection)url.openConnection();
+        // Analyze response
+        // Verifies HTTP status code and the Content-Type
+        Assertions.assertEquals(200, conn.getResponseCode(), "Bad HTTP status received");
+        Assertions.assertEquals(
+            "text/plain;charset=utf-8",
+            conn.getContentType().toLowerCase(),
+            "Response content type is not TEXT PLAIN or UTF-8"
+        );
+        // Get the content and check installation status
+        String diagram = getContentText(conn);
+        Assertions.assertTrue(
+            diagram.contains("Installation seems OK. File generation OK"),
+            "Version diagram was:\n" + diagram
+        );
+    }
+
+    /**
+     * Verifies that the Monaco Editor webjar can be loaded
+     */
+    @Test
+    public void testMonacoEditorWebJar() throws IOException {
+        final URL url = new URL(getServerUrl() + "/webjars/monaco-editor/0.36.1/min/vs/loader.js");
+        final HttpURLConnection conn = (HttpURLConnection)url.openConnection();
+        // Analyze response
+        // Verifies HTTP status code and the Content-Type
+        Assertions.assertEquals(200, conn.getResponseCode(), "Bad HTTP status received");
+    }
+
+}