Make test utils public, so other Its plugins can reuse them

Change-Id: Ieaa41761937add6ad42773fa5be968266c09db02

BUCK

diff --git a/BUCK b/BUCK
index c5d6009..8d14006 100644
--- a/BUCK
+++ b/BUCK
@@ -33,6 +33,7 @@
     '//lib/powermock:powermock-module-junit4',
     '//lib/powermock:powermock-module-junit4-common',
   ],
+  visibility = ['PUBLIC'],
 )
 
 java_test(