AU: Start an UpdateAttempter unit test suite.

This increases the UpdateAttmpter line coverage to almost 40%. More is needed
but this is a good starting point / checkpoint. Also some UpdateCheckScheduler
unit test cleanup.

BUG=6243
TEST=unit tests, gmerge on device

Change-Id: I39c80de3f18095c4a28cb36ab868bed6d7073824

Review URL: http://codereview.chromium.org/3259011
diff --git a/SConstruct b/SConstruct
index abf5253..09a6566 100644
--- a/SConstruct
+++ b/SConstruct
@@ -259,6 +259,7 @@
                             tarjan_unittest.cc
                             test_utils.cc
                             topological_sort_unittest.cc
+                            update_attempter_unittest.cc
                             update_check_scheduler_unittest.cc
                             utils_unittest.cc
                             zip_unittest.cc""")