AU: Add unit tests for the Terminator class.

Also, improve the implementation so that the Terminator can be initialized
multiple times.

BUG=6243
TEST=unit tests

Change-Id: I8d7f1a2007ac28f8c79506f46d628654413b6607

Review URL: http://codereview.chromium.org/5104007
diff --git a/SConstruct b/SConstruct
index 97cc6e3..d036745 100644
--- a/SConstruct
+++ b/SConstruct
@@ -308,6 +308,7 @@
                             split_file_writer_unittest.cc
                             subprocess_unittest.cc
                             tarjan_unittest.cc
+                            terminator_unittest.cc
                             test_utils.cc
                             topological_sort_unittest.cc
                             update_attempter_unittest.cc