devtools/v23: Improve the regression test.
devtools/v23: Improve the regression test.

This change makes some big changes to the way regression tests work:

1) Instead of comparing various old binaries against each other the test
   compares the code in the client against various old binaries.  This
   is nice when you want to test your change to ensure that it is compatible
   with previous versions.  This makes the regression test potentially
   useful in presubmits too.
2) Instead of comparing many dates at once, this change defines several tests
   vanadium-regression-test-X where X is in [1,3,5].  This tests the head
   binaries against those from 1, 3, and 5 days ago respectively.  This is nice
   because when a test fails you'll get more information from the name of the
   test.
3) Instead of assembling a random set of binaries we test several reasonable
   sets.  For examples we'll test old agents against all new binaries, all
   new binaries against an old agent, all new binaries against an old agent
   and device manager, etc.  I have specified initial sets in this CL, but
   these will need to be refined over time.

Change-Id: Ib6c74b4a123933459855a6a171f927f193fca92b

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools