Change vomtest to use reflect.Value instead of interface{}

This gives us better control over the exact value we're expecting
to decode for the decoder tests.  Also added helper methods to
vomtest.Entry to return hex and binary bytes, to simplify the
tests.

I've also fixed some issues wrt type registration for the error
type, but that's caused new failures in the new vdl value reader
tests.  The new raw bytes tests are also failing.  I'll look into
these tomorrow.

MultiPart: 1/2
Change-Id: I49665aeeefc2db47c9349eb54387e61b57fa82c6
25 files changed