Allow to use new acceptance-framework artifact

Extend gerrit_plugin bucklet with new variable, so that plugins can
use it in standalone and in gerrit tree build modes.

Change-Id: I6551d992770babaeb7a9b6b92c53f6da362c9b28
diff --git a/gerrit_plugin.bucklet b/gerrit_plugin.bucklet
index d4edb7f..2c374c8 100644
--- a/gerrit_plugin.bucklet
+++ b/gerrit_plugin.bucklet
@@ -49,8 +49,9 @@
   '-XdisableCastChecking',
 ]
 
-GERRIT_PLUGIN_API = ['//lib/gerrit:plugin-api']
 GERRIT_GWT_API = ['//lib/gerrit:gwtui-api']
+GERRIT_PLUGIN_API = ['//lib/gerrit:plugin-api']
+GERRIT_TESTS = ['//lib/gerrit:acceptance-framework']
 
 GWT_DEPS = [
   '//lib/gwt:user',