Help BUCK to grok which sources are tested by the unit tests

By adding the 'source_under_test', BUCK can now produce a
code-coverage HTML report for the plugin's unit tests

Change-Id: I6f93bdc145fa8bae6c926c52bff57ef49eb8feba
diff --git a/BUCK b/BUCK
index 3aed93a..081ad6f 100644
--- a/BUCK
+++ b/BUCK
@@ -21,6 +21,7 @@
   name = 'cookbook_tests',
   srcs = glob(['src/test/java/**/*.java']),
   labels = ['cookbook-plugin'],
+  source_under_test = [':cookbook-plugin__plugin'],
   deps = [
     ':cookbook-plugin__plugin',
     '//lib:junit',