update_payload: Modify payload_info_unittest.py

This patch modifies payload_info_unittest.py to run in the unittests and actives
it in the run_unittests script.

This removes the dependency to the chromite utils used for unit testing and
creates small similar tools to capture those behaviors.

BUG=chromium:865845
TEST=unitests

Change-Id: Ib931831ce9c4b0ae3fc2f62972db82ec2ec5897f
Reviewed-on: https://chromium-review.googlesource.com/1144643
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
diff --git a/scripts/run_unittests b/scripts/run_unittests
index c8e713d..0d301ba 100755
--- a/scripts/run_unittests
+++ b/scripts/run_unittests
@@ -25,4 +25,6 @@
   python -m update_payload."${filename%.*}"
 done
 
+./payload_info_unittest.py
+
 exit 0