update payload : Add unittest for paycheck.py

Use generated sample payloads to verify and apply each payload.

BUG=chromium:1028646
TEST=sudo FEATURES=test emerge update_payload
TEST=./generate_payloads

Cq-Depend: chromium:2401388
Change-Id: I2b817c4b71edf4cc6bd36d9ee021366818a42ebb
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2401389
Tested-by: Vyshu Khota <vyshu@google.com>
Commit-Queue: Vyshu Khota <vyshu@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/scripts/run_unittests b/scripts/run_unittests
index 0d301ba..db5ed73 100755
--- a/scripts/run_unittests
+++ b/scripts/run_unittests
@@ -26,5 +26,6 @@
 done
 
 ./payload_info_unittest.py
+./paycheck_unittest.py
 
 exit 0