paycheck: unit tests + fixes to checker module

This adds missing unit tests for the checker module, bundled with fixes
to some bugs that surfaced due to unit tests. This includes:

* A fake extent (signified by start_block == UINT64_MAX) that
  accompanies a signature data blob bears different requirements than
  previously implemented.  Specifically, the extent sequence must have
  exactly one extent; and the number of blocks is not necessarily one,
  rather it is the correct number that corresponds to the actual length
  of the signature blob.

* REPLACE/REPLACE_BZ operations must contain data.

* MOVE operation validation must ensure that all of the actual message
  extents are being used.

* BSDIFF operation must contain data (the diff).

* Signature pseudo-operation should be a REPLACE.

BUG=chromium-os:34911,chromium-os:33607,chromium-os:7597
TEST=Passes unittests (upcoming); works with actual payloads.

Change-Id: I4d839d1d4da1fbb4a493b208958a139368e2c8ca
Reviewed-on: https://gerrit.chromium.org/gerrit/45429
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/scripts/update_payload/payload-test-key.pub b/scripts/update_payload/payload-test-key.pub
new file mode 100644
index 0000000..fdae963
--- /dev/null
+++ b/scripts/update_payload/payload-test-key.pub
@@ -0,0 +1,9 @@
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvtGHtqO21Uhy2wGz9flu
+IpIUR8G7dZoCZhZukGkm4mlfgL71xPSArjx02/w/FhYxOusV6/XQeKgL3i8cni3H
+CkCOurZLpi2L5Ver6qrxKFh6WBVZ0Dj7N6P/Mf5jZdhfvVyweLlsNK8Ypeb+Razf
+rsXhd4cy3dBMxouGwH7R7QQXTFCoCc8kgJBTxILl3jfvY8OrNKgYiCETa7tQdFkP
+0bfPwH9cAXuMjHXiZatim0tF+ivpkM2v/6LTxtD6Rq1wks/N6CHi8efrRaviFp7c
+0mNmBNFaV54cHEUW2SlNIiRun7L01nAz/D8kuoHfx4E3Mtj0DbvngZJMX/X+rJQ5
+cQIDAQAB
+-----END PUBLIC KEY-----