veyron/runtimes/google/ipc: Add missing vom registration in test.

Without this commit, if the test binary happened to depend on any
package that did something along the lines of:

type T struct{}   // i.e., named type for an empty struct
vom.Register(T{}) // i.e., register that type

where T did not implement security.Caveat,
then TestDischargePurgeFromCache would fail because the vom-encoder
would encode an alwaysValidCaveat (also an empty-struct), while
the vom-decoder would try to decode into T, which does not implement
security.Caveat.

Long story short, this missing vom.Register call was an oversight.

Also, included unrelated log message fix.

Change-Id: I0eef28091d7504cfe919535c4545ae6d802e5ba2
diff --git a/runtimes/google/ipc/full_test.go b/runtimes/google/ipc/full_test.go
index 03a3d5a..718b9f8 100644
--- a/runtimes/google/ipc/full_test.go
+++ b/runtimes/google/ipc/full_test.go
@@ -1236,4 +1236,5 @@
 	blackbox.CommandTable["runProxy"] = runProxy
 
 	vom.Register(fakeTimeCaveat(0))
+	vom.Register(alwaysValidCaveat{})
 }