PluginIT: Add assertion about plugin's indexUrl value

Assertion about the version is also missing, but this will need some
refactoring in the test and will be done in a separate commit.

Change-Id: If793ff7216844b7f39333d64c294b7e88756120d

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/plugin/PluginIT.java

@@ -58,6 +58,7 @@ public class PluginIT extends AbstractDaemonTest {
       assertThat(api).isNotNull();
       PluginInfo info = api.get();
       assertThat(info.id).isEqualTo(plugin);
+      assertThat(info.indexUrl).isEqualTo(String.format("plugins/%s/", plugin));
       assertThat(info.disabled).isNull();
     }
     assertPlugins(list().get(), PLUGINS);