Annotate abstract test classes with @Ignore

According to I1a9b6ed53 Buck's test runner doesn't skip abstract
classes, but will if they are annotated as @Ignore.

This may not be true any more; Buck seems to be marking the classes as
NOTESTS.

Mark them as @Ignore anyway, even if it's only as documentation.

Also make classes abstract.

Change-Id: Ifc8deef60037bbf0b446542498ba9055037bf5d9

gerrit-pgm/src/test/java/com/google/gerrit/pgm/init/InitTestCase.java

@@ -15,10 +15,12 @@
 package com.google.gerrit.pgm.init;
 
 import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
+import org.junit.Ignore;
 
 import java.io.IOException;
 import java.nio.file.Path;
 
+@Ignore
 public abstract class InitTestCase extends LocalDiskRepositoryTestCase {
   protected Path newSitePath() throws IOException {
     return createWorkRepository().getWorkTree().toPath().resolve("test_site");