Do not load detached plugins during tests (#2829)

* Do not load detached plugins during tests.
Also warn about plugins which force us to create classes.jar.

* Porting POM fix from #2795: we must allow snapshots from repo.jenkins-ci.org so as to be able to test upstream component PRs.

* Fixed failures in tests which assumed detached plugins were being loaded.

* jenkins-test-harness 2.20

* Unreproducible test failure on CI, probably due to timing.

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -194,8 +194,10 @@ public class ClassicPluginStrategy implements PluginStrategy {
             baseResourceURL = resolve(archive,atts.getValue("Resource-Path")).toURI().toURL();
         } else {
             File classes = new File(expandDir, "WEB-INF/classes");
-            if (classes.exists())
+            if (classes.exists()) { // should not normally happen, due to createClassJarFromWebInfClasses
+                LOGGER.log(Level.WARNING, "Deprecated unpacked classes directory found in {0}", classes);
                 paths.add(classes);
+            }
             File lib = new File(expandDir, "WEB-INF/lib");
             File[] libs = lib.listFiles(JAR_FILTER);
             if (libs != null)
@@ -669,6 +671,9 @@ public class ClassicPluginStrategy implements PluginStrategy {
             z.add(mapper);
             z.execute();
         }
+        if (classesJar.isFile()) {
+            LOGGER.log(Level.WARNING, "Created {0}; update plugin to a version created with a newer harness", classesJar);
+        }
     }
 
     private static void unzipExceptClasses(File archive, File destDir, Project prj) {