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/PluginManager.java

@@ -952,6 +952,7 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
      * with a reasonable up-to-date check. A convenience method to be used by the {@link #loadBundledPlugins()}.
      */
     protected void copyBundledPlugin(URL src, String fileName) throws IOException {
+        LOGGER.log(FINE, "Copying {0}", src);
         fileName = fileName.replace(".hpi",".jpi"); // normalize fileNames to have the correct suffix
         String legacyName = fileName.replace(".jpi",".hpi");
         long lastModified = getModificationDate(src);