Mark greenmail as testonly

The greenmail library is only used in integration tests and in the
acceptance framework.

Change-Id: Iec369bd57a146c0d23d7514fd3cdfe477fe9d6e3

lib/greenmail/BUILD

@@ -2,6 +2,7 @@ package(default_visibility = ["//visibility:public"])
 
 java_library(
     name = "greenmail",
+    testonly = True,
     data = ["//lib:LICENSE-Apache2.0"],
     exports = ["@greenmail//jar"],
 )